Thanks both.

My concern is that the flue is a similar perhaps slightly bigger than
a letter box in cross section and conventional stainless liner will
not be possible. So some form of coating might be required.

The other option might be the construction of a separate flue that
runs up the side of our property though the neighbours might not be
too impressed!

£3000 would make it out of the question...so we may just have to buy
another electric fire/heater!

Thanks.