Been looking at the heating/hot water wiring, no particular reason
other than the boiler seems to take up to 5 mins to fire when CH/HW is
activated. System working OK other than that.
(...)
Can't really make head nor tail of the wiring especially as there is a
relay to confuse things, any bright sparks (or plumbers ;o)) seen
anything like this?

My guess is that the relay is probably a 5 minute delay timer. Probably
installed to prevent short cycling. Alternatively, the lot was wired up by a
cretin.

Christian.