Otherwise I am out of pocket?

As no-one else has commented yet and others know more than me - I would say I am pretty sure that the judge can issue the judgement for the tenant to pay the £355 back to you. I also think you need to get the judge to write this in a way that satisfies the deposit scheme - and that the latter will then allow you to claim this. Others can explain this better.

I'm no lawyer but petty sure the court can award costs and legal fees and baliff's fees, also any rent due if they stop paying at the end of the tenancy.

This very bad advice from LAs and Shelter so often leads people into large debt and makes it impossible to rent again.

Also no reference from you.

I successfully claimed court costs, bailiff costs and even locksmith cost (who had to be in attendence on the day according to the court order) against a bond last year.

The tenant protested about the locksmith charge, stating he left the key in the house the day before eviction. Which he did. But he didn't tell us, so we had to assume the property was still occupied at the time the bailiffs turned up (and we were therefore not legally allowed to enter our property until this time and date).

Any additional cost the tenant causes you, because they are not respecting the departure date on the s21, is worth claiming for.

Shelter advise tenants it is legal for them to stay past the end of the section 21 notice period until evicted by bailiffs during which time the tenant has all the same rights under their tenancy agreement (TA) such as repairs yet they do not have to allow viewings unless the TA states this. The council will likely advise the tenant stay on until eviction otherwise the council can decide to refuse the tenant help with longer-term housing,. But if the landlord starts court action & the tenant doesn't defend their case, the court will likely order the tenant has to pay the landlord's court costs & could instruct it be deducted from the tenant's deposit. This would indicate the Bond Guarantee would cover the same but suggest you read your BG's actual wording and check if it states you need to have notified them within 7 days of an event/relevant change such as serving a S21 or applying to the court for possession. Did you?

Without bespoke advice from a Landlord association or using a Possession company, there is a very high likelyhood that your DIY case will fail.
This is not so much personally aimed at you as new to this for the first time, but due to the endless legal complexities that that have been attached to the Section 21 process.

Any expenses you are out of pocket on can being subsequently claimed by making a money claim online. This will leave them with a CCJ against their name. However as I once experienced, if your tenants decide to declare themselves bankrupt, then you will not end up with anything, regardless of what claims you make.

Although a Court CAN make an award against the tenant to pay the £335 back to you, in my experience they don't. The Court system is very expensive to landlords, but can be free to tenants.

One practical point: include in your tenancy agreement a clause
enabling you to claim court costs and other reasonable expenses incurred in recovering possession where there has been a breach of the tenancy conditions by the tenant.

Its not only Shelter and Citizens Advice that advise s21 tenants to stay till the bailiffs are at the door but their local council too.
All costs and rent arrears until you gain possession are recoverable from the deposit ( not sure if have to be noted on the court order) but you have receipts. If deposit doesn't cover everything then a moneyclaims on line for the difference Plus that court fee