First I'd try and isolate where the drain is coming from. Monitor the current and start pulling a few fuses to see if you can isolate a circuit. I'd start with the alarm, steering lock and power steering which are common areas on these.

I'd also recommend taking out the CEM (Central Electrical Module) which is the fuse box above the passenger's footwell, disconnecting everything and spraying liberally with contact cleaner. Dry it out and put it back in. These often either get water in them or get kicked by passengers in V50s causing also sorts of electrical weiredness.

If the seller knew of the fault and failed to mention it then you do have a legal case. Whilst you could argue that the seller surely knew the fault existed, especially if they'd driven the car recently, there's no guarantee you could prove it in court.

Wrong. In a private sale, unless the seller specifically says 'in perfect condition with no faults', or unless you specifically ask about faults - for example "Are there any faults or ongoing problems with the electrics, battery, etc", and they then lie in reply, then they have absolutely no obligation to you.

Those adds are a real ball ache when writing on a smartphone, jeees, right in but the middle of the text.

Simples, use something with a proper keyboard!

a "smartphone" is ok for talking, texting and sending the odd sms/message and using as a satnav or to find a pub/restaurant/shop/ etc while out. as a means of lettter writing beyond a short paragraph, it is a pain in the posterior.