Some tenancy contracts have a clause that you will co-operate to let new tenants view the flat during a notice period, some don't in which case, you would be in your rights to say it isn't convenient.
However, you can try and negotiate with the LL if your tenancy contract has no such clause, i.e. money back if you co-operate and he finds a new tenant who wants to move in during the notice period.
Btw, requesting entry for viewing potential repairs is not the same as viewing to show prospective clients, so respectfully decline the LL's request.