Can anyone tell me when stored pressured water based extinguishers are to be hydrostatically tested in the Uk and where I can find this information out? Is it 5 yearly or 10 yearly? 

Not at all. Only CO2 extinguishers require hydro testing in the UK & EU, all the other types, being lower pressure, are specifically exempted from pressure vessel safety regulations.

(A technical exception to this is if they are renovated by being shotblasted and repainted, but this isn't usually done with non CO2 types anymore as new equipment prices make it not cost effective)

They do require testing by discharge every 5 years as part of an Extended Service.