OK all. Just got the Defender back from my local garage who took on the job.
Once the windscreen, dash and doors taken out / off we found the corrosion had extended to the angle welded to the top of the bulkhead that folds inwards to support the dash / air vents etc.
We got a complete new full width bulkhead repair without air vents and Stuart cut off the old top section just above the drip channel behind the bonnet back edge. He then cut the new panel to suit to save messing around with removing the brake servo etc to use all the new panel which, frankly, wasn't required.
He had to fabricate the supprt for the dash and air vents and weld this to the new panel. We also took the oppurtunity to replace all the wiper linkage which had also suffered!
Paint doesn't match up but it looks a cracking job and should last a few years!