Interesting comments again.
Nope, sink plunger wouldnt have worked on the original gunge because it was pretty disgusting. I dont know what my wife has chucked down the sink when doing her oblutions and havent asked. Neither did some commercial drain cleaner shift any blockage.
Im pretty sure the inside of the waste pipe was probably very gungie all the way to the main waste but it isnt now the snake's been down it. Yep, the anti syphon valve on the trap was also gunged up and as I said in the original post, I suspect that was stopping the water draining away after the grunge was removed.
As regards plumbers, the problem is a number Ive called previous dont want to come out to small jobs. I can understand that if you're putting in £500 to £1000 bathroom jobs etc but that doesnt solve my problem.
To put things in context, when I was working, I always used to get tradesmen in to do jobs, even internal decorating because Id rather spend my down time doing stuff I want rather than going DIY. Now Im on my pension, Im more inclined to do jobs within my capabilities.
Ive always used a local lad and always try and support the local economy. He's normally very good if a little slow coming out. I phoned him a few times to come out to the toilet but got no response. So, that made me have a go at a relatively simple task. I dont consider cleaning out a waste trap a job to call out a plumber. I wouldnt dream of doing serious plumbing work in our gaff and will continue to call my plumber (although if he doesnt respond next time, I'll have to find another reliable guy or gal).