For completeness I thought I'd post how I fixed this.
First I cut the pipe off using a multitool. Before cutting I pushed a cotton wool ball down to just below the cut. After cleaning the cut I took off the back wheel and liner, disconnected the other end of the return oil pipe and blew my airline down it to fire out the cotton wool and flush the pipe out. Top tip.. the pipes cross over in the sill so check the diagram carefully.
I took the cut pipe to a hydraulics place in Watford (http://www.customhose.co.uk/) who put a solution together for me comprised of an 18mm to 1/2" BSP single ferule compression fitting, 1/2" x 80mm stainless BSPT pipe and a 1/2" BSP to 5/8" 60 degree fitting to match the car. I sealed the taper threads with Loctite anaerobic sealant. Access to tighten the compression fitting is limited - I just about got a crows foot in using wood blocks to hold the pipes apart.
The second photo shows the pipe fitted and tested. If you do anything similar i recommend buying a bag of 8mm scrivets as you'll probably need to replace some. I also bought some M5 stainless bolts to refit the mudflap as the old ones were shot. Looking at why this happened, there is a drain hole in the tray under the windscreen which just drips down the bulkhead and down the pipes soaking the foam (and the mudguard brackets 🙂 ).