No access? ................ I'll bet it's under the patio :o(
Bound to be the cloth too....... usual culprits are J-cloths and disposable nappies!
Ok .......... you could try a rubber sink plunger, but it's probably too small.
I've resorted to this before ............. you may not want to, but ........ I've started, so I'll finish lol
Take something like a broomhandle, put some rags (a good handful) into the bottom of a strong refuse bag, stick the handle in right to the bottom, tie or tape the bag around the handle, use it as a mega-plunger................
You'll have to guage the amount of rag to get a tight-ish fit..........
Get a good action going ................. ooo-er ............... it should do it :o)