I have got a generator (Generac BSP7500LE) connected to my house (Via a
proper transfer switch!)

When running from this generator, there is a very noticeable flicker on the
lights all the time.

I have a tester, and it tells me the voltage is 230v and 49.8Hz when the
generator is loaded at about 10A - what am I getting this flickering on the
lights, and can I do anything to stop it!?

I have three UPS' on various computer equipment, and they seem happy with
the generator's supply.

Sparks...