I have an 8 year old (little used) and in very good condition Honda 2113 hydrostatic ride on mower with grass collector.
It has had a new battery but does need its starter 'freeing' off (again)

Wondered how much it would be worth as going to advertise.

I would be better off with a strimmer for the weeds. The mower only got used 4 times last year.

Its hard to tell without knowing its true condition. As a rough guide for you though, I would expect £400 - £500 as a "Part Ex" to a dealer or anything up to £800 as a private sale. If it has starter motor problem you can quite easily loose £100 off the above prices. However, these things are always hard to judge without seeing it, as "little use" can also mean "Seized" & "Rusted" depending on where it has been stored, along with the fact lots of parts could now be starting to perish due to age and lack of use such tyres and belts etc etc.

If you have an overgrown area of land with lots of weeds I would look at something like a DR wheeled trimmer which are much more pleasant to use than a hand held strimmer / trimmer / brushcutter on larger areas of land. See the following link for more info.....

DR Sprint Wheeled Trimmer - 20", 5.5HP with Recoil Start - Lawnmower World