hi, please quick expert mudcat advice required..

I just wasted another hour being passed around various polite but very useless BT call centre departments !

I've got an extension cable wired from downstairs to upstairs, about 10 to 12 metres long, wired permanently through walls and floors.
Its a standard UK 4 wire cable.
The problem is the plug on the downstairs end is broke and needs replacing.
How easy is it to buy and replace.
Can I just cut off the old plug and get a new one that just crimps on, or would it need soldering and be best done by a BT technician
[at probably exhorbitant call out fee]?

also the cable is at least 10 to 15 years old, so even if I can fix it easily myself,
is it still up to fit spec for the purpose of modern digital Gap compatible phones?

cheers mates.