Quick update. I did get my hand down the drain, as everyone suggested, thanks for that one :) Only got out the stuff at the bottom of the drain as the angle at the bottom meant that you couldn’t really get your hand down there. Used some big massive drain coil or what ever you call it and couldn’t clear it, so had to call someone out.
Managed to get it done this morning for £75 all in. Took him about 2 hours to clear it, unhelpful neighbours contributed to this time by refusing the guy access to their back gardens or not being in. We have a shared manhole cover about 5 houses down but the guy couldn’t determine the direction of the water as he couldn’t get access. (so at least the previous owners hadn’t covered ours up).
He said that he was 100% sure that the blockage wasn’t ours as it took him 25 metres of hose to clear it. Joy. so I’ve just paid to clear someone else’s crap.