Old Pot Drainage Gulley

Mornin all , I'v been laying some 4 inch plastic drainage pipes in the garden and back yard . I'v laid all the pipework , which is for rain water drainage only . I now need to connect the pipework to an old Clay gulley thats under the kitchen window , it has a stone hopper head above the gulley which I'd like to keep . The Gulley itself is clay or stone with only the ONE outlet opening to it , I don't know the size of this outlet but it only feels about 4 inchs . I have a gulley with 4 inch INLET and OUTLET openings . The only thing i'd like to know is ""What size will the old pipework be I have to fix the new gulley too "" and It will probly be an old clay pipe . " So will i have to get some fitting or other to connect the NEW GULLEY to the OLD PIPE . I know it's a very long drawn out question . but I'd like to be prepared before i smash any old Pipes or Fitting's . Thank you all very much . ###

I forgot to say that the back of the old gulley is again'st the foundations of the house , and the left side where the OUTLET hole is . Is under some very big steps which I can't move ,It is very tight against the side of the steps , and so can't see what's there . I was hoping someone might be able to tell me what the gulley might fix onto , or if I would need to get some sort of connection to join the two together . Thank you ##

Photos before you smash anything would be a big help...

pipe

Now, if we actually knew WHAT you were working with, and its sizes, MAYBE we could give an answer.