Well, I bit the bullet and bought a pair of the elbow fittings off Ebay for £12 each.
When they arrived, they turned out to be bends with a much larger radius rather than elbows and they didn’t take 1/4″ (6mm) pipe ; as the bore measured 6mm, I assume that they were actually for 5.5mm pipe.
Anyway, after getting no response to my emails from the seller to see if I had been sent incorrect items, I decided to ream the bends out with a 6mm drill (by hand) and, after completely knackering one of the bends, I finally got one to work ; with a bend made with my trusty pipe bender at one end (in lieu of the knackered unit) it all fits in nicely.
I am a bit worried that reaming out the bend might have made it a bit fragile (despite loads of solder to ensure joints as strong as possible), so I intend to split the copper pipe and install a bit of rubber pipe unseen somewhere to, hopefully, help stop any vibration fractures.
Recommendation : do not buy the £12 so called 6mm elbows off Ebay (unless you are using 5.5mm pipe).