Radiator replacement - should we go for higher BTU than needed?

[Madrab]

Would a gap of about 10-15 cm be enough you reckon between sofa/radiator?

More than enough - the radiator works by it heating the air it draws in from below and circulating that out the top and out into the room, it won't lose a lot of it's efficiency being close to the sofa

But without the TVR it was a struggle to get this.

No but I bet it got there quickly .... used to have a TVR Chimaera

 

[denso13]

quite hard to know if the radiator was not good enough (could do with a bigger one though) or if it was just the lack of TVR that made it worse.

You have worked out the requirement and should be able to tell if it has enough output.

If it is too small and poorly sited then it won't heat the room sufficiently, just the same as any other design of radiator.

A radiator behind the sofa will work but you will lose some of the heat so size accordingly.