My thanks to Mike and other forum users, with your help over the years I've managed to keep on top of the simple stuff with our Powermax 185 (now 15 years old). A local guy (South Wales) specialises in servicing Powermax, and thankfully he's done our more serious servicing etc.
Problem just now is when drawing hot water at the bath, with tap fully open so max hot flow, there's an almighty banging coming from the boiler and pipes, especially noticeable when felt on the pipes to the thermal mixer valve, though I guess that could be simply transferred from elsewhere. This banging doesn't happen when we shower (mains pressure showers) or run other hot water taps.
There's no noise when not drawing water, ie the boiler fires up ok and heats the water without complaint, so I'm guessing this is not kettling.
It sounds like a big airlock type of banging. I'm guessing the pipes can no longer supply hot water at full flow rate - is that likely to be what the problem is ? Are the pipes likely to be scaled up after 15 years (the water here is very soft, but after 15 years i guess there may still be deposits)?
Update on this - I took a punt, and replaced the thermostatic mixing / blending valve (P757), and this has fixed the problem. The failing valve had only been fitted about 9-10 months ago, so I hadn't initially thought it would be the problem. Previous valves have lasted about 7-8 years, fingers crossed the new one lasts at least that long too.
Update 23 Sept 2019 - problem has re-appeared, very loud banging when drawing hot water to bath or other taps......will order a new P757 mixer valve (existing one has only been fitted one year....) and with luck that'll fix it.....