On Wed, 13 Jul 2016 15:13:38 -0000 (UTC), Tim+
wrote:

snip

My background 'fear' is there is nothing wrong with anything and that
the upper limit stat has just got weak?

Possible but given that it's a safety device probably not wise to assume
it. ;-)

Quite, and why I've not left it bypassed. ;-)

Is there any chance of localised overheating in the boiler due to an
airlock in the top part of the boiler?

I don't *think* so. The design looks very much like any air would
travel up and out by natural means, let alone because of the pump
pressure / flow.

Without sufficient water to carry
away the heat an air pocket trapped at the top of the boiler could
(theoretically) cause localised overheating.

Understood. Unless an airlock was causing a lack of movement the side
of the heat exchanger the overheat stat was fitted ... I'll have to
have a better look first.

Of course good design and pipe outlet positioning should eliminate this but
without seeing your boiler I can only speculate.

I'll take a pic the next time I'm round there.

Cheers, T i m