A new axle frame needs to be fabricated and a set of wheels obtained. I used old Airfix/Dapol tender wheels from their BoB plastic kit that were left over when I motorised that model and fitted metal wheels to its tender. The wheels should be the Bulleid-Firth-Brown (Boxpok) style but plain disk wheels were supplied with the Airfix kit. Other disk wheels sets could be used of about 14 mm diameter.
Probably the easiest frame manufacture would be tin plate bent into a U shape like the old Wren Bulleid tenders, but with a 3D printer to hand I designed and printed my own.
A NEM pocket coupling is contained in the end of the frame by means of a dovetail slot - just like the latest Hornby models. It is a tight fit not needing glue.
In the photo below the finished tender assembly is hooked up to a 'Hatchett' Mk1 coach. The new frame is just visible between the wheels. If I had thought about it I could have designed the frame not to show at all and spent more time designing in brake gear but as the GBL model overall does not have the fine details of the latest Hornby model then it is ok as is for me. The tender will be repainted along with the engine when I have motorised it.