Not an exact science then......as it seems personal preference comes in to it also. Thanks for all your help. I see that nobody thought tiling straight to boards was a good idea and so I could discount that tiler straight away.
I understand the argument on "you get what you pay for" but, as a customer, I have to disagree that it is a rule. More a guide, as it often doesn't work out that way. I recently had a plumber (allegedly a proper firm etc.) do some terrible work for quite a lot of money. More than the better plumber who rectified it would have charged. I also had a shower room fitted by a big firm in Manchester, paid a lot of money and got an awful job (blocked soil pipe etc backing up to 2nd floor bathroom just one of the problems). I could have paid a lot less on both occasions but went for the more expensive options, as they appeared to be experienced, reputable etc. I have a few examples of this principle but don't want to bore everyone (ok, too late for that).
The proof, I'm afraid, is in the pudding, not the quotation. I just have to use my best judgement as to who will do the best job (and seek advice from tiler's forum). Obviously price comes in to it also but I certainly don't want a shoddy job after shelling out £850 on tiles.
In this instance, Tiler 1 is quite young and Tiler 2 is very experienced, well known to the several tile warehouses locally and was a recommendation from someone who had a lot of expensive tiles fitted in a big extension. He also isn't the cheapest, that is Tiler 3 and I have now discounted that guy. I think the difference in price is just their way of pricing. I may be wrong but wouldn't know until they did the job. I would say that tiler 1 was on about plastering the (new plasterboard) walls before tiling, which I didn't understand, as it reduces the weight the boards can hold but I don't know, maybe there was a good reason.
Thanks to all again for your advice. Much appreciated.