David Williams wrote:
Just installed a new Triton T300si (one of those with a remote heating
pack).

Its delivering barely over 2.5l/min on half heat setting = just about warm
enough for a shower. Pathetic by any measure.

It takes 4100 Joules to raise 1 l of water 1C.
To raise 1l 20C takes 4100*20, or 82KJ.
82KJ/9.5Kw = about 9 seconds a liter, or 7l/min.

This is raising 20C, if you're going from 10C to 50C, it'll be 3.5l/min.
To hit 2.5l/min, you need to be raising the water temp 60C or so.
Unlikely.
Sounds like something is wrong.

Can you measure if it's using the right amount of electricity?