Thanks for the replies and useful information. We haven't finalised the route yet but are leaning towards overnight from Hull to Zeebrugge (to cut out my least favourite part of the drive, from north to south England), then on through France, northern Italy, Slovenia, Croatia and Serbia. The Italy-Greece ferry seems a bit expensive and as Laurie says above, the mountain route through Greece may be a problem in January. I just have to get an insurance policy that includes Serbia.
We did a different route a couple of years ago, in the summer - Dover to Calais ferry, then through Belgium, Luxembourg, Germany, Austria, Hungary, Romania, crossing into Bulgaria via the Calafat-Vidin bridge. My favourite part of the drive was after leaving Germany (a lot of roadworks on the autobahns meant two lane traffic, with our small car either stuck in between HGVs or being flashed by impatient shiny BMWs and Audis. Hungary and Romania were very pleasant driving, and much friendlier and cheaper when stopping to sleep or eat. Coming back we went through Slovenia which was fantastic, and crossed into France further south which proved a nicer drive than more of Germany, Luxembourg and Belgium.