It was a really simple thing - it was just the pressure. However, as we've never used a pressurised system before hubby hadn't the faintest idea (oh, and we don't have a manual for it). The engineer spent longer drinking his much-needed coffee than he did fixing it. He was really pleased, he'd had a heck of a day thus far. He thought it had to be something simple, as he was the one who serviced it less than two months ago.
It turns out that it's almost certainly Dolphin's 'fault'. They had to remove a radiator in the bathroom to re-site it and put in an additional towel rail, which meant the pressure would have dropped, and not being specialised plumbers they didn't go to the boiler afterwards and bring the pressure back up. So it's been slowly 'ailing' since they went. At least we know that's almost certainly what has caused it, thus we don't have to worry about it just happening again, or wondering whether something is fundamentally wrong with it. And hubby has now been shown how to fix it should it happen again - why do this fixes always involve grovelling on the floor? Thus, I can't do it myself!!!
So two relieved people and a happy engineer - hot shower time soon methinks.
We did enquire about replacing the combi-boiler with a non-combi kind, but apparently that is more than a little expensive. So I don't think we'll be considering that route.