The reason that you have not seen any diesels advertised is that none have yet made it onto the British register. Should start to come about this winter...but the initial cost is high however to think of it slightly differently you can pay £15 to £20k for an ordinary ovewrhaul or £35k for the diesel conversion. I suspect when clubs think about it they will like the idea. I also don't think that the lower operating cost will be passed to the hirer straight away.
My hypothesis is that diesels will start to encroach slowly (Dunkeswell flying club being one of the first as they are owned by the same people as are doing the UK conversions). When the first conversions start to come down in price the market will dictate that many clubs will want the diesel. There is a critical event horizon where the development costs are recouped and if enough are sold before this drags the developers under it will be a success. What I can't see is many private owners or groups buying them.