The cistern on our 30 year old close-coupled WC cistern is cracked at
the corner of one side and the font and leaking. This will undoubtedly
mean a new WC but I am wondering if I can make a temporary repair to get
us over the next two weeks.

Whilst a repair should be fairly simple, so is replacing the cistern. I'd be
tempted to do the job properly right away if you can get hold of a
compatible cistern. There shouldn't be any need to replace the business end,
unless you had your heart set on a change of style.

Christian.