Hi Macc
If you are lucky there should be a label on the cover of the IM or if it is
a metal cover it is sometimes stamped into the cover wall.
This gives voltage watts and length.
In your post you say it's a 2.3 kw so you may have already found the cover
with no length info'.
Then you do have a problem.
Most top entry heaters are around 27" (the old heat rising thing so the
deeper it goes the more water it heats) however certain cylinders which are
designed for dual heat (central heating boiler and electric immersion
heater) are restricted internally to length of heater.

Something you could try is removing the 'stat from the heater if it is a
long thin brass stat (around 13" ) then it can't be an 11" heater.
The down side to this is ring,red-ring and the like made heaters with a std
6" stat (silver coloured about 15mm dia)so if you come across this you are
back to square 1.

Maybe others may have more advice.

HTH
CJ