Plot twist - the heating engineer has said he may arrive next week (why are they so mysterious?).Mkay, stopped masturbating long enough to concentrate a bit.
First question, are you sure the smaller rads will provide enough heat? Secondly, do you have a crawl space? How are the rads connected now? Just pipes along the wall?
Looks fairly straight forward to be honest, need more info tho.
Where do you live? Happy to pop by and have a look to see what's what.