Not posted in a while, sorry..
Basically I have a Marshall TSL60 head with a broken reverb tank, everything else works perfectly.
I'm downsizing to a smaller house soon and have a JCA20H that it much more suited to home playing, so the TSL ought to go.

How much should I be asking for it and how much could I expect taking it to a shop? A lot less?

I'm in the UK, if that effects price a lot!
I suggest you get the reverb tank fixed. It would increase the street value by a lot. Things don't look as good if even a less important aspect is broken.
Reverb tanks are not a big deal. It is probably 1 of like 3 things to fix and none of them are super expensive. Use the Cool Look Up link in my sig to get an idea of what they go for on the used market. I'd buy a TSL with a broken reverb tank if I was in the market for something like that. Not a bad amp at all.