If it's the block that bolts onto the cylinder head that holds the Thermostat
This part is still available from Citroen, it's an EARLY type housing.
This is the same part for ALL Aluminium blocks for Series 1 Peugeot 106. This is why your dealer is struggling to find the parts for your 1.0
As all 1.1 and 1.4 alloy block saxos have the "later" type housing.
if it's part "1" then it's £183 from the dealer including the VAT. you'll probably get an equivalent online for much less.
The other part I think was less than a tenner when I looked part "10" that holds the thermostat in place.
It's still readily available from the dealer, the parts can still be produced as they are not "NFP".