The Brew Cabin

I have been browsing your site now for a couple of weeks and as I do my confidence seems to be growing. My problem is simple enough - New house with V-large back garden with a slope down towards a 8m flat behind house. Usual Clay soil, so water is collecting at bottom of slope on a heavy downpour or a heavy snowmelt. I have grasped your advice on putting in a 80mm drainage pipe at bottom of slope and have identified the combined manhole which has an additional spur for which I can fit into. Do I need a trap before manhole section to stop bacfklow or smells getting back into garden from waste water?

You only definitely need a trap if you're connecting to a Foul System, which should be a last resort, anyway. If you're connecting to a Surface Water system, there's no need for a trap, but you won't do any damage if you fit one. :)