going with motorised
thanks again for all the input folks, it was great to get a few opinions. on balance it looks like motorised valves are worth it for the convenience. our plumber says they have a life of 3 or 4 years, he's been switching through all brands trying to find a good one and hasn't found any yet. but as 3CC says most failures are a minor issue.
on the idea of putting manual valves around the motorised ones, plumber said it wouldn't be expensive but it would be impractical in terms of the space needed.
the views here have eased our fears a bit, so thanks very much!