My upstairs cold taps are fed from a tank in the loft that I would like to get rid of. If I connect the main directly to these taps, bypassing the tank, is there any danger that the increased pressure could cause leaky joints in my 40 year old pipework?

Yes it could depending on the rating of the existing piping as in how much pressure the pipes can take when the taps are turned off.

Other than that you would also need to check the taps including the washer to make sure there not going to leak with the increase.

I'am guessing that you already have a tap at the main that turned off all the water to the whole house , though I would place one between the Ground floor feed and the upstairs feed just in case , though I would guess one is already in place somewhere by the upstair tank in case the tank started leaking.