Thanks
@terryplumb and everyone else for your advice. No rads have been removed/altered since system previously all worked well. To be honest, I did not check all the other rads to see if they were fully heated. I will check though later today when the heating comes on to see if all the rads fully heat up. At the moment, we only have the heating to come on for a couple of hours in the morning.
My gut feel is that once the heating has turned off via the room stat, and then the room thermostat later triggers the heating to come on when the temperature drops below the set temperature on the room thermostat, the 3 furthest rads from the boiler struggle to heat up.
I will hopefully be able to confirm this later this evening.