Holding deposit

Tax Planning and Financial

A bit of advice please;

I will be renting out a 1 bed apartment at £480 per month. I am doing my first viewing on Tuesday from a perspective tenant who has shown considerble interest despit not having actually seen inside the apartment. I have held back advertising the property ( each ad is appx £60)as its only a couple of days until the viewing and if he is interested then the ad would have been unnecessary. What would you consider would be a fair holding deposit whilst I check out the prospective tenant etc) if he should turn out to have poor references I would have delayed my advert and therefore delayed getting a tenant.

Is there a normal % used as a holding deposit?

Thanks
Alex