I have another unit which is in better nick. The batteries have started to corrode, but the circuit board looks not too bad. But where can I get the batteries? The part number throws up a very similar batter, but with only 3 connections instead of four.

Electrically, there will just be the usual 2 (positive and negative) connections on each battery but some have additional pins for better anchorage. Usually any extra pins are connected by a PCB track so it's possible to make a different pin configuration fit with some coper wire and a bit of fettling. It should be possible to find one with the same configuration though. What's the spec' and part number printed on the battery?

There are two batteries marked as follows:

VARTA
2X3.6V 140 mAh
2x3/V150H Ni-MH
103 —>+

TPS will still sell you a newer one of the correct spec, if needed. The sirens are all pathetic - I activated the chirp just so I knew the poxy thing was doing something for the expenditure! You also need a different nut to fit it as the thread is different 🙄

I had to replace the plug and wiring as the corrosion had eaten the pins and plastic - it did make fitting the new siren easier as the OEM wiring is very short!

I'm struggling to find anything quite the same from any of my electronics suppliers. I'm sure that particular battery configuration used to be quite common but it looks like nobody stocks it any more.

I think I would probably just check the dimensions and make my own battery 'pack' instead, using something like this (assuming that has the correct dimensions). You'd need to tape (or, preferably, heatshrink) 2 of those together and solder wires to them instead. Don't solder directly to the battery though; some batteries have been known to explode when exposed to excessive heat from a soldering iron.

Still think you should follow my example +++

I have! :)

I found this battery too, but the pins are the wrong orientation and there should be 2 on each side. I don’t really think I can be bothered with making up a battery pack (I’m not the best with a soldering iron!) and I’m not sure that it would fit into the horn if I did. I think it’s all too much trouble for a feeble chirp that no-one would pay any attention to.

It's potentially a cheap (or chirp) fix, assuming there's no other damage. But I agree, probably not worth the effort.

I cut my old one open too when it failed but that was largely out of curiosity. After determining that it wasn't worth repairing, I swiftly binned it and replaced it. Replacements units were readily available at the time though; I might have been more inclined to persist with the repair if they weren't. In any case, I like chirps; it's a helpful confirmation that all the doors are fully closed and locked as you walk away (since it won't chirp if any are left ajar). So I'd still repair or replace if possible.