Gear changing was bad - worse after oil and filter change

[sssam]

OK - the code from the service book is

ABZ DNW DTD.

The £300 included a cheap tyre and a hose which needed replacing. Other than that it was £37 for the filter, £24 for the gasket, £56 for oil plus VAT plus an hour's labour - so that seems reasonable?

I'm at a bit of a loss for what to do... Is it worth spending another £200 (or whatever) on taking it to the ZF specialist? What sort of money do you reckon the whole car is worth otherwise? It's in good shape, full service history with service book, cambelt done at 80k, now got 135k on the clock, MOT to end of year. Body is in good shape. There doesn't seem to be much point throwing more money at it if ultimately it needs a new gearbox for a couple of grand - which I'm not prepared to spend.