In my loft I found a bit of a leak.

The hot and cold to the shower both have a in-line stop tap and then a non-
return valve / check valve. (30+ years old)

The stop tap is weeping a bit and other parts are a bit crusty - I think I
should make up a couple of new lengths with new valves (for what they cost)
and splice it into the existing pipework.

Any recommendations on what fittings are best for this job - bearing in
mind it is hidden away in the loft?