You shouldn't be overly concerned about doing damage to the boiler.
Excluding the earth and neutral connections, all the wires behind the FCU are live, or switched live.
If you make an incorrect connection with one of these wires, the worst that can happen is that the boiler will be on, when it should be off.
If you accidently touched a neutral to a live, then it would be likely that a fuse will blow.
If you accidently touched a live to an earth, your RCD would trip.
But given the rats nest of cables behind the FCU, there is a fair chance that moving it, could have disturbed the wiring. Checking all the terminals are tight and there is no loose wiring, would be prudent.
