Cost of replacing radiators?

[summerdays]

All that happens is the more water you have in the radiator, the slower it is warming up and cooling down. We have half victoran cast iron rads and half modern thin ones. Cast ones stay warm for 2 hours after heating is off whereas the modern ones are cold in 30 mins. This could be seen as less efficient as it is harder to warm the house for a limited time (say first thing before going out to work). But overall I dont think it makes much difference.

I have an old radiator in the bedroom and it's still warming my pj's when I go to bed even though the heating has been off for a while.