Would a BG engineer be classed as a CH engineer...
Only by BG as marketing, to the gullible. They're heating technicians; some are bad, some are good. BG want to sell you a power flush.
we had them over few days back and they mention the system being block by 'sludge deposits' which could one of the main reasons for the rad not heating up.
Do you think it would be worth having the system flushed or even a magaclean filter of some sort installed?
Probably, but you can't get much flow rate through a micro-bore system and you need a high water flow rate to entrain any loose particles. You might need to take radiators outside and flush them through with a hose. A magnaclean filter would help protect the boiler from sludge deposits. Flush the system and dose with inhibitor. Get quotes from local heating contractors, other than BG.