Jeff. The valve can be repaired. Remove circlip and replace the 2 "o" rings. They've just gone hard with age.
A light sand with 600 w & d paper whilst it is apart. Don't forget to assemble with waterpump grease or equivalent.
Heater handles' are hard to come by. You've got the tempory/permanent fix.
A knob off of an old stove may do.
Normally only 2 cables on Belletts. I see you have the complete heater unit out. There's the main air intake and the
other is the direction door for the demist or cabin flow.
If I get a chance on the weekend I will measure the cables and advise.