@Folle
You are quite within your rights to say no.
We lived in a private let house, in the early 2010s for several years. We allowed 9 or 10 sets of people in over 5 or 6 weeks, (who were looking to rent it, as we were leaving.) We spent about half an hour - to an hour - with each set.
We told them the area is nice, the house is snug in winter, and the neighbours are pleasant, and basically 'sold' the house for the landlady. We gave them coffee and biscuits, and entertained the kids while they looked.
We also left nets and curtains, a couple of curtain poles, carpets in 3 rooms (where there was none before, only a concrete floor,) new toilet seats on both toilets, a stand-alone bathroom cabinet, and some patio furniture.
We also spent some 25-30 'man hours' scrubbing the place from top to bottom... both when we moved in (as it had been left a shithole by the previous tenants,) and also just before we left, we spent another 25-30 man hours cleaning it.
We replaced curtain poles and toilet seats etc, etc, etc, because the others were broke/hanging off. We must have spent £700-800 in the 3 years we were there on making the place look nice. Including painting and putting carpets down in several rooms. We also kept the garden nice (back AND front.) And, as I said, we were very friendly and helpful with prospective tenants.
For all our trouble and effort and always paying the rent 3-5 days EARLY, the landlady (who was as nice as pie to our face for the whole 3 years,) took £450 out of our £700 deposit, for 'rubbish removal, tidying the 'messy' garden, and for the 'broken' worktop in the kitchen, and 'broken' downstairs loo door (both which were broken/cracked when we moved in!)
We couldn't prove her wrong/that she was bullshitting because we stupidly took no photos when we moved in, OR just before we left. And we were so worn out at the time (with work, and the move, and other issues,) we couldn't muster up the energy to fight it.
But after that I swore if we ever rented privately again, that hell will freeze over before I do a THING for a private landlord or landlady again. I will never ever spend any money on the place for sure. We weren't expecting to get a medal for it (as she never asked us to do anything,) but it would have been nice to have got the whole £700 deposit back!
It was utter bollocks that the garden was a mess and that we left rubbish! The house - and garden too - was WAY more appealing and tidy when we left, than when we moved in! 
AND I will certainly never let anyone look around while we still living there. Wasting my time and effort and coffee, milk, and biscuits etc... Next time, (if there IS a next time) I will say a big fat loud NO.