Boiler Problem -advice Please!!

MyRubyRed

Guys, my 74 yr old mum had a boiler and 2 radiators (with thermostats) fitted via WarmFront Grant in April. Since then there have been ongoing problems with the pressure dropping and one radiator that just will not heat properly. I have had to bleed radiatiors bi-weekly to get rid of air and top up pressure almost daily now. Engineers have been out on 4 occasions to date (another due tomorrow). Initially they said a tap was dripping (not), then they said it was a leak and injected leak detector/sealant (still no difference). One radiator now heating at top but then cooling down almost straight away. Thermostat on that is fitted ti incoming flow and I can feel the red hot water in the incoming supply but it just doesn't seem to reach the radiator itself. Could the fault be on the thermostat/valve/radiator?
She's had combi boilers for years and never had any problems until this new one (ideal h30) was fitted. At wits end as she's constantly sitting in the cold and getting no where with Warmfront
Any help appreciated