mmzz wrote:
I guess it will need emptying in a few years. Its supposed to be the
sort of 80 quid a year call out and pump thing, but I suspect I may
simply get in with a spade and wheelbarrow and dig out most of it and
stick it on the compost heap one day...

is there any reason why this shouldnt be done?

The inside of the tank may well be anerobic, and harmfull to health.
I would not go inside without an exterior airline, and two similarly
equipped people on the outside.