Rajah has been using his litter tray now for all his toilet needs. I think he would use the bath or basin given the chance as they still hold an intense fascination for him. Last night just before I went to bed, I found him in the kitchen investigating the sink, which fortunately had a cutting board in it, and to which I added a couple of mugs just to make it more awkward. He does seem to like the much larger cage bottom that we have put his litter tray in, as it is deep and catches all the bits that he throws out.
I can no longer blame hubby for the ghastly smell in the toilet, as 9 times out of 10, it will be Rajah who is to blame. The bathroom seems to be the better option for Rajah's toilet, because the kitchen/lounge is very busy, with the dishwasher and tumble drier going all the time, and the tv is on perpetually, so the quieter environs of the bathroom suits him better.
Thanks all for your help regarding this issue. Hopefully it will continue to stay resolved, and I won't have to deal with any more nasty surprises.