Ok - so the beast refused to start today.....

[Polo6n1600i]

I've been using the mk2 this last few weeks, only going back to the Passat maybe once a week. Went to it on Friday morning, normally when I unlock it the headlights come one, but it failed to do this. Tried to start it, very slow turning of the starter - but eventually it kicked into life. On the dashboard the steering wheel stayed red & there was an exclamation sign beside the cruise control. After about 1/2 a mile both lights went out. Checked everything over, no lights left on in the interior - aftermarket stereo, but it's got an Aux loom built by myself to stop any drain issues (and it's not been a problem since I bought it over a year ago)

Parked it on the street on Friday night - didn't use it yesterday - but went down to it this morning and it's totally dead! Not even enough power to unlock it!! So used my key, got the drivers door open & checked under the bonnet - multi-meter showed 6.83v at the tang under there... So pulled the battery off the mk2 golf and connected it to the under bonnet + tang under there. Unlocked the car with main key, then cranked it over - started without an issue.

I'm thinking it's the battery at fault? While it was ticking away I saw it charging on the multimeter & on the app I have on my phone for OBDII faults etc. A steady 13.3v even with everything electrical turned on.... (Voltage fluctuated when items where turned off too - at the battery it showed more like 18.8v!!??)

I'm having trouble trying to find a replacement battery though? Any suggestions as to where to find one?? Looks like it's the original VAG Vatra battery (4F0 915 105E -92ah 520A DIN 850A EN/SAE) so 11 years old - please don't say it's the alternator!!!