I have am Ideal Mexico boiler but some of my radiators never get beyond warm, whilst others are hot. I have tried shutting down hot radiators, bleeding etc but to no avail. The radiators in question are old, should I replace them. I have two newer radiators and they get very hot, very quickly.

The age of a radiators in good condition should not have anything to do with how hot they get. The lines feeding the radiator or the radiator itself might be restricted with debris. You would have to take them apart to check for this or just replace them since you have already replaced others and seem satisfied with the results.