Sewage smell from Washing machine trap?

[Cornish Maid]

For last couple of months, sometimes when our washing machine does its first rinse/empty, there is a very noticeable sewage smell from the outlet. We have an open upright stand-pipe type, with a u-bend trap at the bottom, like this:

http://www.screwfix.com/prods/18640/Plumbing/Wastes-and-Traps/Washing-Machine-Trap-40mm

Our drainage goes to septic tank, which we had emptied 3 months ago, and the guy rodded the drains at the same time to ensure no solids trapped in the pipes. Our utility where the w/m is located is next to our bathroom, both on the ground floor. Toilet is "downstream" of the w/m outlet plumbing. I took the w/m "stand-pipe to pieces, and the u-bend is clean, no sign of any blockage or debris and contained water, so how is this smell getting through? It is possible that the u-bend is syphoning out and therefore running dry? If this was the case, I guess there would be a smell all the time, but its only evident when the w/m drops its first lot of water and subsequent rinses seem to "flush" the smell away.

Any suggestions appreciated.

Thanks

 

[adlplumbing]

sounds like u lossing trap seal fit hepworth stright trap to it and this should solve the problem

 

[Nige F]

means a Hep V O waste valve instead of the trap/standpipe - make your own standpipe . try a dishwasher/washmachine cleaner too - and clean the soap drawer