I've changed the freewheel on my road bike to give me some lower gearing. This means the chain needs a link adding back in so that its slightly longer. Its a sram chain, and I have the links and pins I removed when I first fitted it. Is there some trick way of doing this? I have a link breaker tool, but that doesn't really help because it seems impossible to get the pin square in the link hole and wind the tool as well. Anyone any ideas?

You can't put the old pin back in unless you want to really tempt fate. Best result (short of the new chain idea) would be an additional/ powerlock, although that's going to extend you two links.
If it's a Shimano chain they make special pins for joining vs what's in the chain, is that what you can't get working?

These days you can't simply splice in a new section of chain with a chain tool like you used to before Hyperglide. But the solution is simple enough.

Add a section of chain, as little as one inner link, using a connecting link. There's no rule that says your chain can't have more than one of these.

with a chain tool you can add half links .. as I'm supposing this is a single speed ,
they[kmc?] make half links in 3\ 32 and 1/8" width,

if you leave the end of the pin still in the back outer plate of the chain, you can
press the pin back in.. in the future , don't push them pins, all the way out.

You can press in the old pin and use a center punch to expand the end and rivet it together. Not the best solution, but I have done it and it worked on a 9sp. DA chain.

WARNING- this advice is perfectly OK and correct when applied to single speed bikes, but totally inappropriate and possibly dangerous when applied to multi speed geared bikes with Hyperglide or any brand of gated shifting, as the OP's bike seems to be.

Thanks for your help, looks like an extra connecting link is the answer