The system pressure will rise whilst system is running , this is normal.
Pressure should be set at 1.2 bar
If yours is already 2 bar, then pressure relief will kick in when it rises to 3 bar.
Nothing to do with the leak on the divertor.
A cupboard would have no effect on the boiler cycling at all, modern boilers do not heat up the rooms they are within. a pipe stat combined with a frost stat powered from within the garage would suffice.
The white plastic pipe (condensate) has to be insulated against frost and has to be angled downwards along its length. The copper one would have looked better following the line of the stone wall. Does look messy does'nt it.
You have a problem with yellow pages, or just cannot afford it.
You come across as the son of Agile.
And the Blending valve can be purchased through most plumbers merchants.