Well yes that's inevitably going to continue blocking up. Down to poor workmanship. Looks like you will have to get back onto the insurance and company.
Vaillant ecotec pro 28
Vaillant ecotec plus 825 or 832.
Both stainless hexs
Personally if your only hot water demand is for a kitchen sink and maybe a bath in the future a 32kw boiler is overkill. Even the 25kw boiler will be ample for your 3 bed semi heating demand.
Drayton mid position head. Like for like wiring. Take head off first tho to check the spindle moves freely... Only left and right a fraction on these valves. Quick release black lever to take off.
Open 'all' the radiators...vent all the air out of radiators, when system is cold put pressure upto 2bar then isolate boiler....no drop over a day or 2 then boiler is fine, when isolation is turn back on and there's a drop then it's your rads or pipework.