I think that the cold spell has revealed that my battery is past it's best. It's a LR branded (Varta) YDG500130, but I cant see any date stamp so have no idea how old it is. It would appear to be OEM and a lead acid. If it's the original it's done well at 7yrs old, which is feasible going from my last D3.
I am considering staying with lead acid rather than AGM and from a search on here, this looks like it should do the job;

https://www.tayna.co.uk/EA1000-Exide-Premi...7488.html.
It says 017TE, L-353mm W-175mm H-190mm which from what I can tell will fit.

Unless anyone has any alternative suggestions.
Thanks.

I replaced my Defender’s battery last year - it was 20 years old after all and I had it on a conditioner or solar charger when the vehicle was not going to be used for long periods. I was advised to go for a battery with the highest CCA - cold cranking Amps - when I replaced it. Fitting a high CCA battery has transformed cold weather starting. They are available in standard form lead acid batteries as well as AGM batteries and are no bigger than a normal battery in size. 2020 Pangea Green 1st Edition D240 New Defender 110 is here and loving it
2018 Melting Silver Mini Countryman PHEV - soon to be replaced
2015MY Corris Grey SDv6 HSE Dynamic, the best car I have ever owned, totally reliable only a cou0le of rattles in 3 years, now no longer in my care
Also in my garage is a 1996 TDi300 Defender 90 County HT made into a fake CSW

Got you. The Amp/hour is higher, but the CCA is actually 50 less then, although the LR fitted says SAE and the Exide measures it in EN. Looks like I need to do some more research. Many thanks.

OK, 950 SAE is the same as 900 EN according to google
Just wondering if it's worth looking for something with a few more CCA. Living in the East of England, we don't seem to have too many really cold mornings,and I only have to get going early once a week.