So would you say an 18" thermostat is correct for a 27" element?

I would.

http://www.plumbworld.co.uk/277-14813

Would be simple if the tank accommodated this! What's the longest element
that can be put into a tank?

The problem is that top mounted ones aren't very efficient at heating the
bottom compared with a side entry. You can find longer ones, though, which
may improve matters a little.

However, you can't beat a good old side entry. Your tank will accommodate
it. You need to drain down and then drill the correct sized hole with a hole
saw. The element can then be installed. This will provide more hot water
than any top mounted device.

Christian.