Just thought I'd belatedly report how this went. N.B. measurements are
fairly rough...

Using 5 (five!) akasa 2500 80mm PC fans (£5 each) set in a line in an 82cm
raised MDF box with the gaps between fans blocked off.. Running at 12V
increased the temperature drop accross the radiaor by about one third
(approx - as measured by digital probe thermometer wedged gainst
pipe/radiator using pipe insulation). IE it effectively turned the 1000mm
radiator into a ~1300mm one. The room temperature went up in line with this
(about 2.5 degr), Running at lower voltages appeared to have a roughly
proportionate effect - eg about 15% improvement at 6V.

I didn't get to do any measurements with just 2x2500 fans, But I have the
impression this would have raised the temp by more than 2/5 of the above one
third but still some way short of the 33%,

I also ended up with a 3 x 1000 RPM box, which seemed to raise the output
silently by about 10%.

I tried blowing the rad with 10" deskfan at various angles to the radiator.
The rad was a double, with the inner faces and convection fins blocked off
by the radiator sides and the back blocked off by the brackets. For this
reason I wasn't suprised to find this had little effect, maybe 10%, the
shelf fitted a few inches over the top might not have helped.

So I ended up with the small fax box in use all the time under RAD2 (room
has 2 rads) and the 5-fan box on demand, including 20 mins after CH is
switched off to extract the last dregs of warmth.

One thing I would like to have tried is something like a thin mains tower
fan on it's side blowing air up through the radiator from the bottom - I
think this might work a treat!.

Skeggy (on Googlr as server has lost my post).

Thats a shame, I meant the OP to go as a reply against my earlier topic,
basically about trying to use a box containing a row of 12V PC fans to get
more heat out of a radiator to get the room remp up a bit.