The solenoid is available separately. Don't bother with GSF/ECP, goto a local battery/alternator/starter place and just get a new solenoid.
You can remove it on the car, its three flat blade bolts at the end, undo the braided wires and it should slide out, be careful not to lose the spring when withdrawing it.
I got a new one for mine about a yr ago and it was £15 fitted IIRC.