Landlord - "Our plumber couldn't find the source of the leak, therefore you caused it"

I (F,26) moved out of my rental two months ago, but in dispute over deposit return.

I was away for a few days during what I recall to be a wet week. I returned to find a wet patch in the bedroom floor, and damp patch in the ceiling of the room below. Days later the agent sent a plumber that informed me they cannot see where the leak had originated. They came back a second time and lifted floor boards and said there was no continuous leak present, but maybe it's from an external wall leaking in (verbally). Note, where the external wall is, cupboards from floor to ceiling stand. The Landlord admit the leak starts in the cupboard.

The Landlord now wants to deduct nearly £2k for sanding and fixing the wooden floor (where there is a stain of about a foot long). Also for fixing the ceiling in room below.

Surely just because the plumber couldn't find the source of the leak, it doesn't mean I caused it?

Questions: 1. Does the landlord need to show that I caused the leak, or simply that there was leak damage and their plumber couldn't find the source to be awarded my deposit? I obviously can't evidence that I didn't spill water (inside a cupboard), which is what they're claiming in essence.

2. Any advice how to respond to the TDS? What should I say, or list as evidence? Had I known they were to blame me for the leak, I would have arranged my own inspection, but they waited till after I had moved out to send me plumbers report and blame me.

Thanks for any help