Common problem on these.It is usually the the plastic housing that becomes brittle and porous and fractures around the seams after so many years.Not a difficult job to do,but as you have found out ,genuine parts are not cheap.After market,pattern parts are now available at a much reduced price.Never used them myself,so can't comment on the quality.Probably worth a try.The plastic gasket/seal should come with new housing.hth

Not necessary to drain the whole system.Remember to do the work with cold engine.Undo the coolant cap to release any pressure and then refit cap.This will reduce the amount of water loss when dismantling the stat housing.Place a container under the area to be worked on to catch the coolant.Once the job is done,refill with correct a/freeze water mix (50 %)and run engine with cap off untill warm to allow air to bleed out. .Make sure the heater is set to hot and fill as reqd.Let system cool down and recheck from cold.hth