While on the topic of radiators… my eventual plan for the upstairs bedrooms is simply to replace the radiators with slim lined flat panel ones.
Hudevad (first two in top row) do lovely flat panel radiators like the Plan range and they are normally what we specify at work, however they do come with a rather ridiculous price tag.
Alternatives for about half the price include the Vita Plan from Stelrad (bottom). Stelrad has a useful online radiator size calculator which you can download here. Unlike other calculators it takes into account detailed room information such as window size, wall build up, and whether each wall is adjacent to the interior or exterior. Interestingly, most of the existing radiators we have are oversized, however after speaking with the Stelrad technician I learnt that it is ok to oversize them, it just means the rooms will heat up quicker.
In the end I have ordered a Planal Horixontal range from Ultraheat (middle) for the spare bedroom as I plan to get our plumber to install this when he hooks up the dishwasher to make his visit more worthwhile. Ultraheat radiators are the same price as Stelrad, but as they are available in100mm increments, we were able to get an 1100mm long radiator which will fit perfectly beneath the window, at only slightly less the current BTU output. Job done!