Thanks for all the replies and suggestions. First chance to get back on with a proper computer rather than the iPhone which, whilst awesome in any ways, does not always lend itself to lengthy typing sessions!
So as an update.....had already spent ages going over everything, checking, rechecking, triple checking. Called the tap manufacturer to find out any unusual requirements (none). Got advice from my Mum's fella (plumber but 70 miles away). Got advice fro Father-in-Law who did the plumbing. Got advice from you guys. Banged my head against the wall for a few hours.
Discovered the hot and cold feed to the sink mixer fed into the wrong sides so swapped them over but this had no bearing on the bath mixer. Flipped the bath mixer 180 as suggested by my Mum's fella but same problem occurred. Plumbed a spare waterfall bath mixer (which is obviously not designed for LP but was a test) and worked fine despite the poor flow rate...no backflow. So took the mixer apart as I'd tried everything else and no-one could think of anything else it could be. This had been my first thought but wanted to test everything else before dismantling a new product.
This is where it gets....odd! It had a ceramic mixing cartridge - 1/4 turn usual affair. But with it still plumbed into the hot and cold feeds I turned on each iso valve in turn. Lo and behold both hot and cold were able to come through the body of the mixer and mix before reaching the mixing cartridge. The two small inlets in the body of the mixer that go to the cartridge allowed both the hot and cold to flow through - there was no seperation. Took it off the wall and blew through each inlet and spout blocking off the necessary holes to verify this (with my Mrs present and even she could understand what was happening). My first thought had been correct, I assumed, the tap was manufactured faulty.
Called manufacturer and supplier (the supplier was more help) and revealed my discovery. No problem. Another mixer on it's way. Turned up after a week and a half.
Went back home upon it's arrival thinking would all be sorted in a couple of hours. Fitted a couple of NRV before starting just to be sure we'd get no back flow even though I'm lead to believe this is only necessary to prevent contamination of a mains cold supply which wouldn't happen from our gravity tank system. Thought 'I'll just take the cartridge out and have a look before cracking on'. Nothing different to the first mixer. Same mixing of hot and cold in the tap body - no separation. Took said dismantled tap round to In- Laws hoping my Mrs' Dad would say " You numpty, look at this....it works this way!" but, partly to my delight and partly to my sorrow, he agreed that he couldn't figure out how we'd ever be able to alter the mix between the hot and cold.
So now I'm back to square one. Can anyone explain to me how a mixer that mixes the hot and cold in the body of the tap before it reaches the ceramic cartridge would work? I've left it fitted at the moment and all we can get is varying degrees of warm! I'm over the backflow problem....I haven't discovered why this was happening but it has been easily rectified by fitting the NRV's. If I can get the mixer to work maybe I'll go back and try to figure it out but one step at a time.
The only gravity hot and mains cold, Steveb and Diamond, I'm assuming will be the kitchen. As mentioned though fitting a second mixer (a HP waterfall that mathches the sink) didn't cause any problem other than a ludicrously poor flow.
At this point I don't even care about looking a complete idiot if it means I can fit this thing and have it working. The mixer is a Pheonix Pu bath filler/shower mixer (can I put that down here?!). If anyone has fitted one or has any experience of one please let me know. Also I'd like to loose the NRV's as god forbid I have to come back and remove tiles to replace them if they foul up as they're in the wall.
Cheers chaps and any ladies. Hope you're enjoying the Christmas festivities.