Bek is the square metal thing the radiator?
Maybe a couple of inches thick, a couple of feet wide and about 18 inches deep? With some big rubber hoses going to it from the engine?
These are often a weak link in the cooling system and whilst it will not be especially cheap it would not cost the earth to replace (guessing �50 for the rad, free to replace if you have a buddy who is any good with cars!). Or places like your local small garage, or Kwik Fit (altho I hesitate to suggest places like the latter, they tend to rip off people who don't know much about cars).
Keep the expansion tank topped up before you drive it, dont go far, watch the temperatue guage if you do drive it