... I like mooching around decrepit buildings and post-industrial decline...
You'd be easily satisfied in Hull!
swishy is right, though: much the Old Town is beautiful and well worth a wander. Some of the museums are worth a visit too - Maritime Museum (though that's a bit faded and in need of a refurb tbh), Wilberforce House, the Hull and East Riding Museum, Streetlife Museum and the
Arctic Corsair - as is Ferens Art Gallery, if that's your sort of thing. The Deep is also excellent, though it's a tenner to get in and can be a bit crowded at weekends.
There are some tremendous pubs in the Old Town and surrounding areas if you fancy stopping for a pint. My favourite is The Minerva, which does an excellent lunch and a great pint, and is right on the waterfront with a great view up and down the Humber. IIRC a local folk club meet in there on Sunday afternoons, though, so it might be full of men with beards singing sea shanties.
If you fancy getting out of the city centre the Avenues area is very pretty, and there are some great bars and eateries around Princes Avenue as well.