Constant beep and won't run after battery change

[steve_naive]

I'll bold the important bits:

Car hasn't been run for 6 months.

Removed battery 6 months ago. Replaced battery bay (don't ask).

Charged battery last night, popped it in this afternoon, constant beep inside the car, car turns over fine but dies within seconds, everything appears to work (ok drivers door won't open from outside, even with the knob up).

I've disconnected battery to stop the beep from discharging it.

Rear parcel shelf and amplifier are out at the moment.

Bonnet was stuck recently but greased now, just in case it's bonnet switch related.

Need a new battery? Immobiliser? User error?

[steve_naive]

Ok I got the car to turn over and run long enough to reverse down the drive and back. Bit of a lumpy idle but to be expected.

Also inflated all four tyres off the battery so it can't be dead.

Beeeeeep!

[steve_naive]

Got her running and driving, stalled at a motorway junction, so no Italian tune up.

Got home and it threw up the lack of power sensor - have I dislodged an abs sensor when I did the shocks and springs?

Was so perturbed that I'd forgotten about fitting eibach/bilsteins. Wondered why it wasn't skipping about.

The discs have surface rust from standing, you can hear them when you brake, not sure if that could have anything to do with it.

[Mr Tidy]

Sorry I've no idea what the problem might be, but my Z4 had some random electrical issues when the battery died earlier this year. I had to get 3 amber warning lights cleared by my trusty BMW Indy after I had fitted the new one.

I think you need to start by getting fault codes read from the OBD.

Good luck getting it sorted.

[steve_naive]

Cheers Mr Tidy.

I've an Autophyx scanner on the way from Amazon. I don't want to be revving the backside of it if there's an issue.

Neighbour just knocked to tell me I'd left the parking break on - it's still bloody beeping.

Next day for posterity:
Started 1st time this morning, bit of a lumpy idle, reversed and went forward without stalling, even the beeping stopped for a while. The beep is also warbling like a darllek intermittently.

Think I'll add some fresh fuel - it's probably been stood longer than I imagined.

Edit: The beeping has stopped.