That wee thing is unfortunately unobtainium. Its a pressure relief valve that allows coolant to flow from the lower thermostat housing to the water pump if the engine is at high revs when it’s cold before the thermostat opens.
Been looking for a replacement myself. Looks like it can be prised open, but I’ve not been brave enough myself yet to try.
They have now sent the payment email but saying ignore it because "The part is not available" - fingers crossed you order is okI didn’t get that Steve. The site allowed me to pay straight away. I ordered two taking Chef‘s advise. If there is a problem with you getting hold of one, you can get one from me in the mean time when they turn up.