How about the polypipe or polyplumb website? I think they have calculators and guides, don't get bamboozled into buying all their expensive "Proffesional" products though, you can do much better by buying individual components from a plumbers merchant.
It is a lot simpler that many people make out really.
Desp
Yes of course -forgot about that ! doh .......
Probably where i got my info from -was 10 yrs ago .
I used 16mm pex pipe but struggled to find fittings -what is the preferred size ?
thanks desp .........