Thanks, chaps.
After a bit of faffing about with various bit of contoured styrene, I decided it was going to be no harder to construct the louvre apparatus out of strip and rod stock from scratch. So some double sided sticky tape was laid on an old yogurt pot to give me something of a decent size to work and the 11 louvre components attached. Actually getting these equally spaced but not jammed together was a real pain, but eventually I was happy with the result and some liquid extra thin was applied.
The louvre blocks were cut to size - a row of three narrow, 5 mid and 3 wide, then some extremely thin rod stock was applied vertically to replicate the connecting strips. These should be flat but I have had to compromise here for the sake of my sanity - many of the master modellers here may be appalled at my compromise, (and William would probably ace it in 1/144th!) - but I'm not aiming for a Telford-winning model here, there will likely be other compromises later. (The louvres would be ideally formed on a PE brass sheet, but I don't have the facility to do a Harry Woodman on that one.
Anyway, once cut to shape, the item was carefully peeled from double tape backing and tried in place on the radiator to see if it worked. It's a little uneven in the pic, but all will be squared off when properly attached after painting.
Well, I'm happy enough with the result.
Sandy