Get a new seal, almost any motor factors place will have one, even Halfords.
Even getting a new thermostat will not break the bank, they are only about £7 & that may have cause the leak so best to replace it while you have it open.
With the engine cold, undo the hose clips (in fact you may not need to remove them if you can still get at the screws).
Undo the screws holding the top of the thermostat housing on, water may pour out. Take note of how the thermostat sits in the housing, perhaps take a photo with a digital camera so you can refer to it later.
Wipe it all down with a cloth to dry the area, if the old seal has disintegrated and stuck to the housing/body clean it off with a penknife, taking care not o scratch the metal, finally clean it with wire wool wiping around the rim (not inside to outside or outside to in) and wipe over with a cloth.
Fit the new thermostat & cardboard gasket, you should not need any further sealant, refit the lid and screw down again.
Refill the expansion bottle with water, run the engine for 15 mins, check the water bottle as you may have to put more in when the thermostat opened & took the water in. only refill the water bottle when the engine has cooled down, put a cloth over the lid when opening - beware of steam.
Save yourself a £30+ bill, DIY