Yes, it's a common problem Max. So many combis and unvented mains pressure systems around these days. A gravity system often has this problem. A pump fitted in the airing cupboard (on the hot only) would sort it, but it would be cheaper to change the taps ...... again!
I guess your monobloc came with the flexis, and they screw up into the tap body. This makes it very difficult to convert to 15mm copper. Even if you found adaptors, I'll bet the inlet diameter is only 8-10mm........... so you would be no better off.
Clutching at straws ............... maybe an airlock, with insufficient pressure to shift it.
Turn off the cold, open the hot, and apply your face to the spout .......... blow hard......... if it's an airlock, that should shift it.