After a bit of research (a.k.a. Googling), I found Gizzi Erskine's chocolate Guinness recipe, which she has slightly adapted from a Nigella Lawson one. This was a great recipe which made a lot of cake, and the only change I made was to use 85% cocoa content chocolate rather than 70%. Actually I also subbed self-raising flour for plain flour and baking powder, but they are both very minor swaps.
May have lost perspective of portion control with this slice. I may also need to invest in a cake stand.
My cake also took well over an hour to cook, but this is probably because I used a smaller tin to cook it in, so it ended up being deeper than Gizzi's original recipe. But the final result was still excellent. This cake was moist, with a deep chocolate flavour and a slightly earthy hint from the Guinness. And despite containing a shedload of sugar it really wasn't overly sweet at all.
So this is an ultra-fantastic cake recipe, that I would very happily make (and eat) repeatedly.