Water damage from baby sock?

I agree with Ian on the implied duty to act in a tenant-like manner. With washing machine issues, I always let the tenant know - before I arrange a repair - that if the reason for the malfunction is down to the user, they are expected to meet the cost of the call-out.
When underwired bras were really popular (two decades back!) it was often the wire that came loose in the wash and got caught up in the filter. Not saying that the tenant would have caused an event like that deliberately, but it comes down to the user, not the owner of the appliance. I've always been happy to call on the preferred repairer to put things right - so I'm never unhelpful or unsympathetic - and the tenants have all been fine with this approach to date.
Yes, I house them and they use my appliances at their convenience but I don't tell tenants when to go to bed or what time to go to church on Sundays. It follows that I don't supervise their laundry efforts either.
In the event of the washing machine leaking (due to bra wire, sock or whatever), escape of water causing damage has always been covered by my landlord buildings and contents insurance. I just sort it and move on without chastising the normally mortified tenant. Life's too short!

I would say that despite providing manuals and also informing tenants when they move in about washing machine filters, dishwasher salt etc. I think less than 10% of tenants have ever filled the dishwasher salt (I always refill it between tenancies). No one has ever changed a cooker hood filter (and in ducted cases rarely clean the metal grease filter)
Those plastic shirt collar inserts are another thing that often gets stuck in a washing machine filter! Baby socks can get stuck too. If you read the reviews on certain brands, this seems to happen more often with specific models. In this case I don’t think it’s the tenants fault - it’s an accident. Although usually the blockage just results in a machine not draining rather than leaking but proving it was their negligence rather than an accident will be almost impossible.

Looks like you've had some sound advice above. As it's a flat there will be building insurance taken out by the freeholder which can be claimed on for the damage to the flat below (if it is significant enough to warrant such a claim, sharing the excess cost and risk of hiked premiums the following year!). In my opinion, I see this as a accident and a 'reasonable' tenant did nothing wrong and is not at fault. I had a Bosch w/m which kept getting blocked. It was new and under warranty and they found a child's sock stuck in the sump below the drum. They did not charge me for the engineer visits as they deemed it not the tenant's fault if the sock slipped through but did advise her to use a net bag for small items in the future. After the second occasion I told her that if she does not use a net bag she will have to pay for her own engineer to come and unblock it, problem solved! You might find that a useful example to relate to in your situation.

Its not always about blame and getting somebody to become liable but find a sensible solution, learn from it and make sure it don't happen again in any properties we rent out. 😉

Think carefully before you let a tenant tamper with anything that keeps the water in.
We had a tenant who decided to check the filter while the machine was running, with predictable results.
Much as you would like to consider your tenant to be a responsible adult, sometimes the government is right; some tenants are incompetent nongs. The trick is not to let to them.