The thermostat on my Morgan has decided to remain closed

No wonder it was running hot!
I have found a replacement on Rimmers site but I have a choice of 3.
One is identified as Aftermarket for £3.78, another is OEM at £18 and a third Genuine Landrover at £22.34
Rutters have one for £16.44
Needless to say they all look the same although I am reluctant to get the cheap aftermarket one.
Any thoughts?