Will a 2005 Wiseman’s Crossing Shiraz Cabernet Sauvignon, that has been sat on my desk upright and in direct sunlight for the last 4 years, make a nice bottle for Christmas dinner or will it be vinegar by now?

Three issues
1) changes in temperature
2) UV degradation
3) cork drying out and allowing air to pass through

1 will cause the wine to age more rapidly. If it’s really been cooked, you’ll notice more of a brick colour, and perhaps even a sherry-like aroma of oxidation.
2 depends on the colour of the bottle. Most red wine is in green bottles to reduce the effect of UV, and assuming you’re not in a country of high temperatures, this is probably not as much of an issue as 1.
3 May well have a screw-cap, so not a problem. If a cork, see 1 for oxidation.

It’s not an expensive wine (£5-8); I personally wouldn’t risk it, and just open it with supper. If no good, tip in a casserole?

If no good, tip in a casserole? shove the cork back in and give to in-laws

FTFY

Thanks. I assumed it was reasonably expensive as it was part of a gift. Sounds like it might actually be a good one to mull and spice.

If you wouldn’t drink it – don’t cook with it!