So got back from the knockwood weekend all safe and a few days later the tyre pressure alarm went off.
Discovered a very slow leak in the rear drivers tyre and after checking the valve was tight then reinflating it twice over a 2 week period, decided I better pull the tyre off and have a look.
Found this:

Which turned out to be this:

So using this:

Did this:

Then this:

Then this:

And finally this:

In the past I've used the smaller black plugs, but felt the hole left by this rusty nail might be better with the larger plug.
This is the third time I've used these plugs and never had a problem to date that needed a tyre centre to replug for me.
Happy however to hear from BD or similar if there's something else I should do or have done.