When I removed red jacket all seemed fine, nothing melted and by that time we had emptied the airing cupboard altogether, so no tights etc...
(for info jacket back on and cupboard got its contents back now)
Rads seem OK now, but when you say feed AND expansion tank are we talking two different tanks? (sorry humble homeowner here - and blonde one at that!) The water levels in big water tank and what I think to be the expansion tank are OK.
Meanwhile the smell goes on... not as strong that it infiltrates the house any more, but I know it's not right. I know it's strange but i get the feeling it is stronger at the top of the cupboard than the lower half.
The feed and expansion is one tank that has two functions, FEED the system with water, and take the EXPANSION that may occur. The one you want is the smaller tank in the loft. This is for the central heating system. The larger tank is the F&E for the hot water cylinder.
Look for mould/damp as well. THis may be the smell, MAYBE