The natural progression is obviously the T2, but you are far from the only Sony owner who doesn't like the 'upgrade' from the 350/650/950 series to the T1/T2.
You could maybe consider a used 650 from ebay instead of focusing on factory refurbished/ex-display models as you should be able to pick one up for less than £50.
Another option (though not quite as good as it means your lighted case, SD card etc are of no use) is to try the 350. Smaller screen obviously but I'd imagine it should work pretty much as your 650 did? I bought a refurbished one from Morgan Computers last year and it's been fine. They used to have 650's available but they all show as out of stock now, but they do have a silver 350 showing as in stock for £35 delivered -
here.
Another option altogether is to consider trying something like the Kobo Glo, it'll probably take a bit of getting used to, but it's worth having a look at one in your nearest WH Smith if possible. Though if you don't like the T2, possibly none of the new ereaders will be that attractive to you.