In almost all other circumstances that would be a way forward but not in this instance, for me anyway.If you are using a welder with 13A plug then a 13A extension lead is practical. You could make one with 2.5mm which is about as much that will squees in to decent 13A plugs & in-line sockets and a quick check says it is OK for VD at 40m length.
Or maybe better, put in a couple of fixed weatherproof sockets around the area so you are never more than 15m from one when you need it! That way 6mm cable could be used, but it has to be UV proof so not T&E !!! (Unless in conduit/trunking)
I sought further advice from others and there seems to be a consensus.
As the Mig will have very little use and that use spread over a couple of months while learning how to weld before getting on in earnest and repairing and as the Mig will be used at the furthest end of the garden, (50m) for a short time only my intention is to do below.
Remove the 13a plug top supplied with the Mig and replace with a 16a plug.
Make up 50m of 3 Core HO7RNF 2.5mm² as an extension cable with 16a socket and plug at either end.
Replace the existing mounted 16a external socket with one that has RCD protection.
Incidentally both people I spoke to independently were appalled the original installer of the 16a socket relied solely on a 32a MCB for any kind of personal protection and why they didn't fit a RCBO, I'm grateful you mentioned the RCD.
They did say just for now stick to the 32a mcb but when done with all welding to swap it for a 16a mcb.
The 6mm cable that supplies the 16a socket from the CU would have been better run in 10mm but for my temporary welding needs "should" be ok.
Fortunately most of the welding repairs and metal fence add ons will be within a 25m radius of the 16a socket, still a long extension cable but better by half.