All fixed,
For anyone with this problem it's a straightforward job if everything goes smoothly unlike mine.
Turn off the power at the mains, get a bright light that you can leave on to light up the oven, I made the mistake of working in virtual darkness with a penlight.
The screws holding the backplate take the star shaped allen key type heads, not phillips, mine rounded off straight away and had to be tempted out with a drill hammer, long nose pliers etc.....don't try until you have the right head.
4 more different screws hold the element behind the backplate, these were not as bad as the others but were still corroded, a phillips screwdriver got them out.
1 more screw is behind the fan and holds a top centre connection, my original was snapped so to make things easy I snapped that piece off the new one, the 4 screws hold it tight.
Two wires connect to the left, make sure you take note of where they go before disconnecting and make sure they are long enough not to fall into the back of the oven when undone. Tape them up if needed.
I had to pull the element back out to remove the 5th screw connector and lost a wire. If that happens undo all the front side frame screws (same heads as above) and slide the oven out. Remove the back cover screws and pop the lost wire back through the gap.
Check the fan isn't obstructed by the element before you put the front backplater cover back.
Don't put the cover on or push the oven back until you're certain it's connected up ok.
Screw everything back in place, flick the power, turn on the oven and then....It starts working again
Hope this helps someone to do an easier fix than mine.