Thanks for all the comments.
Firstly, the room is never going to get close to winning any efficiency awards, it's got some pretty large windows etc, which I am aware of, but the rads seemed to have a negligible impact. Looking at them with no specific knowledge, they do seem pretty small which is why I was investigating the BTU-ometer thing. I will have a look at the output of similar-sized modern equivalents – didn't think of that.
Excellent point about placing – I hadn't thought of that either. They are not near the windows, so maybe some re-planning will help.
I am very reluctant to have someone round as I fear being told I need new boiler, system, anything else etc etc!