E30 Zone

is it possible to repair this unit?

Do you mean the thermostat, or the valve for the cabin heater?

For the former, just buy a new one; it's not worth risking the life of your engine on a bodge.

For the latter, depends on the issue. If it's a leak, it's normally down to perished o-rings which can be repaired. If it's seized due to fitting the coolant hoses the wrong way round, then you'll need a new one.

The valve for the heater coolant has a diaphram built in that tends to tear with age and lets water leak out through the coil body....this is if it is not leaking at the O rings.
Generally unfixable....you need to get a new one as one from a breaker yard is probably also old and perished.

Is yours not working at all or is it leaking water into the footwell?

I have been waiting to get an a/c system heater valve for 3 months and more. BMW don't have any, a new batch is being made and is due to arrive in Germany on 31/03/14 My name is on one of them. £ 150 thank you very much. Gave up on second hand one as there over 20 years old now.