Caps are not available for downpipes, would suggest removing it and replacing with a bend, however, given the soakaway is pretty useless, you wont gain much. I would suggest, you dig out and redo the soakaway, (so it at least accepts some water), if there is no other option available for drainage.
Then relay the pipe from the downpipe, use a 90° bend at the bottom and take it in a straight line to the soakaway. (Ideally this needs doing in 110mm pipe, but if you want to reuse the 68mm downpipe that will be OK, just make sure it has some kind of protection from anything that could damage it.)