Sime Friendly Boiler issue - radiators hot, sometimes not?

This is a bizarre one (or maybe it's more common than I think!). Here goes:-

Due to falling water pressure all the time, I recently had an external expansion vessel fitted to my Sime Friendly as the internal one was faulty. This is probably not in anyway related to the problem I have at the moment, but I thought it best to give you the backstory.

It's only because I have the Central Heating on more with the time of year that I have noticed this strange issue. Sometimes when I put the Central Heating on, all of the radiators (recently all bled) get hot as they should (based on the thermostat setting) and sometimes not (they are hot at the very top and sides, but very lukewarm elsewhere). This is the case with ALL the radiators in my house (one-level) - it's either feast or famine! Anyone have any idea bout the likely cause? I should point out that I can get Domestic Hot Water without any problem at all throughout this.

Help someone please!