Thanks for the replies.
I ended up going for the Jaegers; I spoke with my supplier on the mainland (none of the shops here carry them in 300W) and fortuitously spoke to the person who handles all their warranty claims; we had a discussion about various makes and reliability and to my surprise, she reckoned that one of their cheaper lines, the Aqua Zonic Eco had the lowest rate of warranty claims of all their heaters during her time working there. Unfortunately, these were out of stock and as I'm limping on one heater AND I'm heading away on holiday soon, I didn't want to risk waiting, hence the Jaegers.
I had an older Jaeger that came second hand with a tank I bought - the new ones although similar look to be much better made, with a very fine heating coil which looks almost embedded into the glass.
Hopefully they'll be OK - they're claimed to be suitable for 600-1000L, depends on ambient/room temp I guess; I'll be away at the coolest time of year, so the house temps will probably dip down to around 10-12 degrees C; I'll run the thermostats at 22 degrees C to keep their appetites and ammonia excretion down, so hopefully the heaters will cope ok.