Yes I know your situation better than most here.
You seem to be doing the right thing being service agent for a few manufacturers. Contact Gledhil also, they don't stock you with parts but you only need temperature sensors grundfos pumps and one control board (though it is very expensive one)
The spin offs from gledhill work are double because there is always a boiler attached. Gledhill customers find that they can get someone to service the boiler but not to fix the gledhill, so they would prefer you as their main fix it man, and you can have all their work, as they will respect you for not feezing in fear at the sight of a gledhill.