don't worry too much about the clauses in the mortgage contract and the covenants in the deeds to the property, they don't matter unless the mortgagee or previous owner (at some time in the past) actually found out and took action. get some public liability and declare your earnings and you're no criminal, only matter is the council who do control use of buildings. if it's unlikely you'll get change of use then as someone said above, don't tell them. do your neighbours a good deal on car repairs and keep the noise down and you'll be an asset to the neighbourhood!