Many thanks Shona, tried ringing them but no response though it is Sunday I suppose.
Rather than leave it to chance I've booked from Perth with a cancellation option so if I can find anything better I can take it up.
Maybe there's a gap in the market for car hire further north, though Inverness would be OK, but starting early from Perth I'll get to Cairngorm top car park by c8am
whereas from Inverness it might be more like 10am.
Freeheel