Thanks to elstaci and zion_black.
Apple have declared my iMac as obsolete and so they've dumped all their stocks of spare parts (they really are working hard at making money from new sales !).
I have found a supplier in Melbourne that has "refurbished" HD4850 units for A$450 which is a lot easier for me than $900 for an upgrade. With labour, I might get the iMac fixed for A$600 which is bearable. If it gets close to A$1,000 I'll look at a new iMac (sorry but, Windows has made life too unpleasant for me to consider any PC) – new iMacs might be announced on 30 October.
I just hope that the old 4850 will last longer than the new unit Apple installed in 2015.
Cheers.