I'm cooking them at 175 degrees (sorry I don't know gas marks!) in a fan oven. Leaving it for 25 initially but takes about 45 eventually! From what you've said about the butter, I think it could be that.
How long are you cooking them for and at what temperature?
Make sure up butter is room temperature, and cream it with the sugar until it goes really pale before adding the eggs/flour. It shouldn't send your hand mixer flying to the side when you hit the butter it should just go right through it easily. You could always warm it slightly in the microwave for 10 seconds if you're not sure of it.
If I ever give mine a wobble half way through cooking the top will look cooked but it definitely shakes under the surface, that's not a problem, just close the door and leave them for a bit longer.
I always cook my 8" cakes on gas 4 for around 40 minutes in the middle of the oven, and my cupcakes on the same temperature for around 20 minutes (bog standard gas oven)
I tried to make some cakes today - one was an 8" Victoria sponge and the others were cupcakes in ice cream cones. Both had the same problem in that when I checked them, the top looked cake like but when it moved in looked almost liquidy underneath! And the seem quite greasy too.
I think it might be due to:
A) my butter isn't soft enough (although it was left put on the side while I was cooking)