Thanks for all your comments.
Roughcaster>> that's great, thanks.
morrik27>> ?? you have lost me, I don't have any external corners so I don't see the relevance, it is just a normal rectangular room!!
joe-90>> I can see how it sounds ridiculous, but, the current walls are not very good and the tiles I am putting on require something more supportive than standard plaster - as even if it is really good / new it will only withstand approx 20Kg per square metre of weight, the tiles I am using weigh about 28-30Kg including the adhesive. However plasterboard will support approx 32Kg per square metre, so my theory is if I screw through the plasterboard and plaster into the breeze this will have the same strength as a standard plasterboard wall. If this was I room I was painting I would certainly have gone with your suggestion as that would definitely be the best option in that case.
Richard C>> The original walls are really bad, but the main reason for the overboard is strength for the tiles, otherwise I would have the walls patched and skimmed as per your suggestion. As for the adhesive, that's great B&Q here I come.
Regards, Lianne