Some of our smaller installations, 4-10 stations, are done with flexible hose and have been fine.
The problem I have found is getting the right mix of flexibility and kink resistance. If you use 19mm hose or bigger it is sometimes quite stiff and heavy and can put undue stress in the fittings if you are not careful.
I have used 1/2" hose and washing machine fittings on the 4-5 station units - I think washing machine fittings are great - so easy to fit and undo.
Ian, do you have a pressure reservoir on the pump set to absorb pressure shocks? Also the newer inverter controllers will alter pump speed to smooth out pressure fluctuations and these can be retrofitted to some pumps.