"Terry" wrote in message ...

"never seen one that did not have that 'overheat' bi-metal disc
cut-out mounted"

The old-type BS immersion heater thermostats didn't have an ‘overheat'
cut-out device. The reason for this lies in the design of the hot
water systems.

Until 1987, the UK Water Regulations only permitted the mains water
to be connected to the kitchen tap and to a cold water storage tank in
the loft. Hot water was usually supplied from an indirect heater
(cylinder), usually on the floor below the loft, with the cold water
feed pipe from the loft tank and with an open vent pipe from the top
of the heater discharging back over the tank. If the electric heater's
thermostat failed, as described here, the worst that might happen was
that steam and boiling water would blow out of the open vent pipe into
the tank. The old tanks were usually galvanized steel, so they could
survive boiling water.

It is a fairly foolproof system and accounts for our renowned ‘English
drizzle' feeble low-pressure showers. On the positive side, the
traditional system made contamination of the water mains by
back-siphonage very unlikely and the tanks served to minimize the
effect of peak-demand flow rates reducing the mains pressure. The loft
tanks might fill up more slowly, but there'd be no difference in the
water flow at the outlets. The water suppliers' pumps could be smaller
and demand was more uniform.

The accident in Cornwall mentioned above happened because, although
the plastic cold water storage tank could have tolerated boiling
water, it was not adequately supported. The plastic tank's sides
softened, distorted and collapsed.

http://news.bbc.co.uk/1/hi/england/cornwall/3035606.stm

Your hot water tanks are connected to the mains water supply, maybe
via a non-return valve & PRV. A water storage tank in a draughty loft
would not be a good idea, given your winter conditions. I've never
encountered pumped well systems here, although there are some in rural
areas.

Since 1987 unvented mains-pressurized water heaters have been
permitted here, to bring us into line with the EEC. The majority of
houses still have traditional open-vented systems. The newer unvented
systems are much the same as your tanks. They rely on three safety
devices on the electric heater (primary thermostat, secondary
resettable cut-out and the temperature & pressure & relief valve) to
prevent the tank over-heating and bursting explosively. The primary
thermostat and secondary cut-out are usually both built into the
immersion heater thermostat. This resettable-type of thermostat has
always been specified for the electric immersion heaters installed in
unvented systems. Some unvented systems have non-standard immersion
heater bosses, to prevent the cheaper, non-resettable type of
thermostat being fitted as a replacement.

"Deteriorated tanks and corroded heater elements have been something
of a
problem here"

You could try posting any query on the US discussion forum at
Heatinghelp.com, ‘The Wall'. There are some very knowledgeable people
there & they seem to have the answers to most such things. In
particular, Larry Weingarten, who posts there occasionally, knows most
things about your water heaters.

We still have the bayonet type bulbs. I think they're OK, but I don't
know any different. Maybe they've improved since you last saw them.

"And yet cars over here use some bayonet (and wire-ended) lamps!"
Maybe Lucas electrics weren't so bad after all then?