Plumber is suggesting replace pipework at a cost of 1k, and that may fix it, and if it doesn’t he can do other pipes for another 1k, and that may fix it, but if it doesn’t, it could be the boiler pump or something else entirely.
2k to possibly fix an issue and that is not guaranteed either. Do you live in a castle?
Do you have plastic pipes feeding problematic radiator?
Looking at some previous posts, some unhelpful posts.
If the boiler has cast iron heat exchanger, then the temperature drop between flow and return at the boiler should be 11 degrees C
SE tended to be similar temperature but difference could be greater but not more than 20 degree C. Look at the nett and gross ratings for this boiler. Gross is higher than nett
Condensing boilers. Gross and nett ratings nearly the same hence boiler is 95% efficient at startup from cold.
Condensing boilers need to have 20 degree differential between flow and return for sake of efficiency ( extraction of heat from flue gases)
Having established the differential at the boiler, balance the system for same differential at each radiator.
If differential cannot be achieved then either pump speed too low to overcome system resistance or pipe undersized or blocked.
Plastic plumbing, the preferred option for most plumbers can block up unless necessary steps are taken. The system water which has metal oxides and metal particles ( magnetite), as it flows through the pipes, will deposit a layer onto the inner wall. In plastic plumbing this layering will be film like and when it dislodges, will flow downstream and can cause restriction and blockages. At times this blockage can be like the leaves on a sink outlet stoping sink from draining and other times a crusty sponge like blockage that can be cleared up if rodded but here pipe replacement is most times the answer. Pressure will not dislodge the blockage even on a short run.
Hope above useful as these are scenarios not imagined but encountered while faultfinding. Have three radiators on a system am working on- two of the rads are 15mm plastic, flow into first two is nice and hot and return is poor hence rads do not heat up and third is 10mm copper micro-bore, further away, plumbed from previous two is poor too because of poor delivery due to plastic plumbing it is fed off. These radiators are in the extension that was added to the house. Rest of the house all copper, no issues.