Sun Wasp Tandem, refurbished, £150 [prov. sold]

Good question, which reminds me that the bike comes with drum brake pads (if I can find the box) that came with the bike when I got it, but that I didn't need.
The rear hub being virtually new, I'm confident it has plenty of life. The front hub (original I think) seems fine too, the pads seems to have a good thickness still (2/3mm at least), but I'm no expert in drum brakes.

ON THE SUBJECT OF BRAKES, IT'S WORTH MENTIONING THAT THE BRAKING POWER IS QUITE LIMITED... IT'S ALL ABOUT ANTICIPATION AND APPROACHING JUNCTIONS / POTENTIAL DANGER / HIDDEN CORNERS CAREFULLY. I WOULD BE VERY CAREFUL ON A STEEP DESCENT FOR INSTANCE...

Saying that, and things put back in context (this is a bike for short / local recreational and slow paced rides,) I / we never had any problem, accident or crash... If you've only ever ridden disc brakes I your life, you'll be surprised! The rear brake is actionable via both handle bar brake lever and foot pedal. When using the foot pedal (which needs getting used to, to find it, and is potentially accessible by both riders) you get a lot more power.

Back to servicing of the bike:
-the headset is fitted with new cones and cups (100 miles max since fitted), I'd be surprised if you had any problem anytime soon, and if you did, last time I checked these Brampton parts were available via tandem.org.uk (haven't checked since purchasing those a few years back)
-the drive train is quite standard as far as I can tell. Cotter pins cranks with replaceable rings (probably no longer made but likely sourceable on eBay / salvageable on old bikes). Again, as far as I can tell, cup and cones BB are British standard. The eccentric bb insert would certainly be hard to find, but being steel not alloy, unless you're a brute, it should last a long as the bike.
-when re spraying the frame, I did not spot any cracks or dents, or any aggravated rust. I can't confirm structural integrity, I'm not a frame builder, but I rode it with confidence... No crash since I got it (the off road incident I mention in OP was just spokes collapsing, we didn't fall... 12 spokes in one go after hitting a massive deep pothole), and the bike was mostly kept indoors or in shed.
-the original rear wheel, which is at BC at the moment (Sorry Jim) needs new Sturmey Archer internal parts. It's the standard AW shell, but, due to drum brake making spacing wider, it requires a longer axle which I failed to source. I left the wheel at BC, and we think it's serviceable. It would be a good thing to do, and I would certainly do it if I was to keep the bike. Stronger wheel (40 spokes) and perfect hub match work front)
-can't think of anything else, but I'll try to answer any question...