On the base of the engine case there is the strainer plate with it's 13mm central nut. Then to the side and across from this is another small plate with 2 10mm bolts fixing it on.

I've looked through the haynes manual, it shows it in a poor picture but does not say what it's for.

can someone advise me what this is for?

The plate off to the side or front of engine is for an oil temp sending unit. There's a special plate with a boss to mount the sensor

Thanks for the info. I assume there is a void behind this so no need to remove it.

The one with a single nut is often called the "sump plate". The oil pickup is above that, and the strainer. If you overtighten the center nut you can bend one of the engine through-bolts, breaking the crankcase inside the engine.

Don't do that. Spec is ~9 lb-ft. Just nine.

The one with two retaining nuts is often called the "taco plate". Above it is the sump, and you can see the bottom of the dipstick through the hole if you take the cover plate and the upper plate off.

If it's not leaking, and you're not replacing the plates to install ones that have a provision for the temp sender, leave that alone.

--DD