Window to Conservatory: HHSRS cat 1 hazard

Have you considered mechanical ventilation to the bedroom ? If there is a chimney breast, or an outside wall, you could install a fan that puts a positive air pressure into the room and this will need to escape via door frames or vents. When I looked into it a few years ago, this was the only solution. Check out http://www.envirovent.com, they have a wealth of advice on ventilation.
In my experience the internal room is often more of a problem for means of fire escape, and I've had to install windows direct to the outside with fire escape hinges.

We've got a student house with a similar layout and the conservatory is SOUTH facing. At the last inspection, there was some mention of the bedroom that opened onto the conservatory getting excessively hot. I pointed out that the students are in occupation from mid/end September until early June, so the peak of the summer is avoided.
We carry out our maintenance during the summer and I have never entered that bedroom and felt that it was hot, let alone excessively hot. So a bedroom that faces north is highly unlikely to become excessively hot, particularly if as you say, there is plenty of ventilation in the conservatory.
As an aside, the Council also wanted a smoke detector fitted in the conservatory even though there was nothing particularly combustible in there, on the grounds that one of the kitchen windows opens onto the conservatory and they said it was a fire risk. Never mind the mains powered and interlinked heat detector in the kitchen and the smoke detectors in the hall and landing providing an early warning of a fire!
We use Aico smoke/heat detectors and the installation manual says don't install them where the temperatures can exceed 40C or be less than 4C - both scenarios could apply in a conservatory but I was concerned about nuisance alarms during the winter. I found a website that recorded daily both day and night temperatures in our location to prove that winter nuisance alarms would be likely.

This was all so ridiculous but the Council still insisted on the smoke alarm in the conservatory despite the clear evidence to support my argument. In the end , I agreed to fit a smoke detector in the bedroom concerned but threatened them with the 1st Tier Tribunal if they persisted with the detector in the conservatory. Then they saw sense and gave in.

I saw a presentation at our local landlords' group given by an EHO from a local Council (not mine!). He said the HHSRS system is supposed to be evidence based. Where is their evidence that the room is excessively hot? They won't have any.
Demand to see their calculations. Band B has a score 2000-4999, whereas Band D is 500-999 so there's been a huge shift. I would say they are obliged to provide their data.
This is an extract from the Landlord Manual part of the HHSRS;
"Local authority Environmental Health professionals undertake assessments and they must decide for each hazard what is:
• The likelihood, over the next twelve months, of an occurrence e.g. falling down stairs, electrocution etc, that could result in harm to a member of the vulnerable group; and
• The range of potential outcomes from such an occurrence e.g. death, severe injury etc."
So the assessment is based on what might happen during the next 12 months which I think gives you an opportunity to take temperature measurements next summer.
The HHSRS Guidance says on page 60 that the most vulnerable age group is those over 65 years of age. You are letting to students. Furthermore, at para 3.05 it says "Overall, the burden of heat-related mortality and morbidity in the UK has been modest, and data to allow quantifiable attribution to dwelling condition is weak."
So if I were you I would;
1. point out that the conservatory faces NORTH.
2. note the amount of floor area in the conservatory that is in the sun at midday in June (presumably, the house obscures some of it which never sees the sun).
3. check out a website that records daily temperatures in Portsmouth to flag up the fact that daily temperatures rarely exceed a given temperature, say 33C and when those occasions occur. Do it for a couple of years. Look at this website;
https://www.timeanddate.com/weather/uk/portsmouth/historic?month=1&year=2019
4. correlate those dates with the 'summer recess' when the students aren't there anyway.
5. offer to monitor and record the temperatures in the room in question over the summer on a daily basis to prove that the temperatures aren't excessive (and linked to the fact that the students aren't there anyway).
6. monitor and record the temperatures in bedrooms that face SOUTH for comparison purposes.
7. if they still won't be reasonable say that you will take them to the 1st Tier Tribunal - it's a pain and potentially expensive but these people are obviously idiots and need to be put back in their box.
8. see if the NRLA will offer any assistance with your case if it goes to Tribunal. Don't hold your breath!
Basically, if you demonstrate you have the facts to show that their opinion is not valid, they might back down because they don't want to be taken to the Tribunal. It is only their opinion and this latest one conflicts with the previous one. I think that is in your favour.
Good luck!

Simon
We see nonsense “over egging” of HHSRS by councils all the time. We can provide an alternate assessment by one of our senior certified HHSRS practitioners and assist with fighting the council. If you would like assistance please drop us the relevant documents to help@landlordsdefence.co.uk or book a free initial call at http://landlordsdefence.co.uk/book