Annoyingly it died at the garage where I was getting the wheel bearing done. Garage is clueless on P38 electrics (like me!)
I've found the EKA procedure and I'm hoping to go down there and sort it out. The garage is charging the battery overnight so hopefully it'll hold long enough for me to sort out the EKA issue and get it onto the guy's ramp so he can change the hub and brakes.
So.. I don't think I'm going to trust that battery any more. The usual online suspects don't seem to have anything man enough in terms of CCA and most of them are at 70 aH which doesn't seem enough.

So, good old Hellfrauds...http://tinyurl.com/zda6j5m they seem to have a reasonable selection. Should I just go with the £99 one? I'm thought I read somewhere that the P38 prefers a lead acid to calcium, but that might be BS!

Try this for size!
http://www.batterymegastore.co.uk/hankook-mf31-1000.html
Marty and I have both used these. Leads are a bit of a stretch, but fits OK. Just have to shave a bit off the retaining strap.

I second the MF31-1000... as Orangebean says - I run one in my P38 aswell...

The say they are a 'calcium' battery, but I haven't noticed a problem with it charging in the P38 (I've got 2 fitted to mine - a MF31-750 and a -1000 hooked up on a split charge system. They've been in there for the past 4 years, and still going strong!

Battery Megastore do next day delivery too if order placed before 1630. You get the added comedy value of watching the delivery guy go a strange colour as they struggle up the drive with a small, very heavy box! As your car's in the garage, you could get the battery delivered there and let them fit it and do the mod to the retaining strap for you. One point to watch is, if you've replaced the standard battery terminals with vertical stud ones like this http://www.polevolt.co.uk/acatalog/BT461P-150.jpg check the clearance to the bonnet on the +ve if you've got a lot of HD cables bolted to it. You REALLY don't want to ground that terminal even when testing to make sure it clears! Don't let it put you off though. The standard connectors are not a problem and clear everything quite happily.

If you can wait till the next day, get the MF31. For the price it can't be beat.

While the V8 will start just fine on a smaller battery, unless you already have a spare battery, its pretty much false economy buying anything else. Both my green and silver ones had/have little ~60ah 600-650cca batteries, but as soon as it dies on the silver one I'll be getting another MF31 for it. Any extended period of cranking and it will start to sulk pretty quickly, or jump starting other vehicles.

Bought the MF31-1000 and its a brilliant battery. I think I bought it from Battery Megastore via Amazon but I also remember a 5% discount code given if you like their FB page.

As already mentioned its a tight fit with the leads but I never had the problem with the retaining bar as others mention . I have a metal bar retaining my battery that is height adjustable and fits fine over the top , so not sure if its the same as other models.

The problem with the retaining bar (on mine anyway) is that it's inverted "L" shaped in cross section so if you don't cut back the vertical part to suit the large battery top you've got a very thin contact edge clamping down on the battery, which has the potential to cut into the top of the battery.

Sounds like my bar is different , its more or less straight.

I think that's the one in there already! So it's probably time for the same again. Thanks folks!