Crazing of glass (plastic) transparent front cover on instrument pod

I have a Nov 2005 reg Focus Zetec (Mk 2 UK model) and there seem to be growing cracks in the glass/plastic transparent screen in front of the
speedo, dials etc.
It has developed slowly, and we were slow to take it back for attention under warranty. Even though we have an 'extended warranty' the Ford dealer tells us that this is just cosmetic and they won't deal with it after the first year. A replacement screen is not available and they would charge about 300 for a replacement instrument unit that would mess up the mileage.
I can get a replacement unit from a breakers yard, but the dealer tells me that it would have to be fitted to replace the whole instrument unit, as the cover is welded in place. I'm wondering whether it is possible to do a decent job of separating the cover and swapping with my installed unit.
Has anyone else experienced this problem, and have knowledge of whether a swap of the screen is a viable fix.
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