Nigel at QRG was suggesting an up-rated matrix and then fitting it in a 6b heater box. Total cost about £250 and a few mods to fit it.

My radiator man (Mansfield radiators) said he could build me one using three row, low height 25 fin per square inch ie the same as the main radiator. Cost £75. I have to say it is now a very very good heater.

The matrix was just a couple of mm thicker but still slid nicely into the heater box. Strongly recommend it.

Pete