Curiousity has got the better of me.......
When I first looked for a replacement expansion tank the one I had in mind was from a Volvo but I hadn't found one when
I was given the Rover 25 one that I ended up fitting.
Anyway I now have a Volvo one (S70, 850, etc), which I have now fitted to my 1850.
(Having been puzzled by Roy's observation of a VW having its minimum level mark closer to the cap than a Rover one,
looking on ebay, I have also discovered that Seat/VW expansion tanks have different min/max marks. Seat versions are lower.)
Some measurements.
Highest point of tank (ie the cap) to minimum mark
Rover 11cm, Seat 11.5cm, VW 10cm, Volvo 9.5cm
Coolant capacity at minimum mark
Rover 650ml, Seat 400ml, VW 600, Volvo 350ml
coolant capacity at maximum mark
Rover 1000ml, Seat 800ml, VW 1000ml, Volvo 1000ml. (I estimate that Roy's is 1150ml)
min to max
Rover 350ml, Seat 400ml, VW 400ml, Volvo 650ml
Total volume of expansion tank
Rover 1750ml, Seat/VW 1400ml, Volvo 1450ml.
Volume of air in tank (coolant at maximum level)
Rover 750ml, Seat 600ml, VW 400ml, Volvo 450ml. (I estimate that Roy's is 250ml)
For those yet to fall asleep, my conclusions and observations
The best position for the expansion tank is as close to the heater blower as possible since you want the tank to
be as high as possible. This rules out the VW tank on an 1850 due to a lack of space. Roy has shown that it is fine on a Sprint.
It is possible to position the Rover tank so that the minimum is almost dead level the (top of the) thermostat housing,
the Volvo tank in same position has its minimum mark nearly 2 cm higher. This height advantage is desirable.
Both the VW and Volvo have level warning indicators, which are easy to wire up.
I think overall that the Volvo tank (part number 3547446) is the best of these three, but there is not much difference really

.