I decided to change the radiator water and pulled off the radiator hose off the themostat housing to dump it.
I found the less than 12 month old housing was crumbling away with corrosion!!!.
So I got another from Repco instead of Autobarn where I got the last one from.
What I have noticed since is brown stains around the join of the two halfs of the themostat assy.
It seems to be also coming out of the bolt holes, could this be residual rust in the holes or is it being caused by coolant leaking internally thru the lower housing thru it being corroded thru on the inside?
The stains are just enough to be worried about.
Any one come across it before?

I am guessing that you are referring to the top hose outlet? Did you replace the O-ring seal around the Thermostat? Did you take the thermo hosing off?

Do you RUN coolant? because it sounds like you are not. I would suggest you drain your radiator and the whole system then run some clean water through it. Leave it for a day and drain again then refill with coolant and water. 3 litres of coolant should do the job.

I didnt change the O ring but most times I dont and if it leaks its normally sraight away.
I pulled the cover off and put it back on again.
The coolant is green and the stains are brown not green plus the coolant was green even before i changed it.