Thanks for all the excellent replies.
I have fitted a humidity detector for the fan. The reason for the mould on the external wall is due to it being an old house with solid walls (no cavity) this means when the temp drops outside the wall quickly becomes cold. When we have a shower the external wall is covered in condensation, the same same effect can be seen on the toilet cistern (ceramic) condensation can be seen on the cistern but only up to the water line where fresh cold water sits.
The problem occurs in other rooms but not to such a degree, it has been increased due to the fitting of double glazing and a sealed multifuel stove in stead of an open fire (but the house is alot warmer now).
One idea i had was an Alde aluminium microconvector (you'll have to google it the forum won't let me post the link to the web page), however i was worried about it being aluminium and problems with corrosion when linked to the copper pipes, also it has 16mm ends and the copper is 15mm. It's designed for a mobile home setup running with some fluid not water.
Another idea was to run a pipe along the skirting, you can see the large pipes used on heating in schools etc, but what is the largest pipe you could link to standard 15/22mm copper piping? it would need to attach close to the skirting for cosmetic reasons,
I don't suppose you have any photo's centralheatking of the solution you did, i know what the copper roofing straps are but what are ft's? Did your solution have the 3 pipes running parallel along the skirting in an S, or was it some intricate pipework creatation?
Martin i did send an email to discrete heating with a query regarding this matter but got no reply.