Paul - have a look at the Seaflex site
although they don't mention it they apparently make the RNLI bladder tanks.
According to Ribraff, Paul Tilley who I can personally recomend makes them which I didn't know! (you learn something every day!) He is registered on here so maybe a PM will get his attention, either that or PM me and I will let you have his number
As mentioned by Alan Priddy, they are difficult to fill, have a shorter life than other tanks, and are expensive!
All the best in your search,
by the way the info came from This thread