Well I'd want to do whatever will definetly work and would be easiest and cheapest, really. But I like the idea of having a plan B.
So am I right in thinking from reading your link (thanks for posting it) that the plan B would be to join straight into the existing acorn T with a fresh bit of normal 15mm hep pipe with a normal hep insert? Yes or copper
So far I've never used hep. I'd have to get some and have a practise.
You could train a monkey in 5 mins....... even know some sparks who can do it😀😀😀
Edit:
Would it? Well that would be the easiest shot surely? I mean I know copper would fit, you can see it does in the picture. Just would be worried about whether it would seal after disturbing old, unusual fittings.