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Much appreciated for all this, will take a look today and either way get this done. Looking at some guides seems very doable too. Learnt a lot working on the car already and still amazed how easy it is to work on. Touch wood it'll continue like this with no disasters!
jambon-
No problem, it's quite an easy job, I did mine after the failure. If you fancy getting a bit more involved then you can preemptively replace the CAS o ring as well, it's quite fiddly without taking the cam cover off, but it's doable. If you want to do the cam cover gasket as well then it makes life easier but costs more money as you need a £30 cam cover gasket and a £5 tube of sealant. MX5 Parts sell all these bits, genuine Mazda parts.
It's worth checking any history and receipts you have with the car though, because if the CAS o ring has been done in the last 50,000 miles then you don't need to do it again.
NurseHolliday
When you replace the rad, it's worth replacing all the hoses too, black silicon is a good option.
You don't have to do all of them, but the ones that are important are the heater hoses, the Cam Angle Sensor seal (CAS o ring) perishes over time and results in oil weeping onto the heater hoses and eventually causing a failure which gushes your coolant all over the engine causing huge clouds of steam, which is what happened to me.
If your CAS o ring isn't weeping now, it will at some point. The inevitable failure of your heater hoses can be prevented by upgrading those hoses to silicon if not the remaining. You can check, yours might have been done already if the previous owner was aware of this issue.