Hi guys to bring you up to date we did a huge investigation and found it!
The house has a solum and we went in there, there was a pipe that had come loose and was completely open letting waste water from the shower and the washing machine etc just pour underneath the house. It appears that during this time something (I can't tell what it even was) has died in the water under the house.
The pipe appears to have been knocked loose either when the boiler was installed, or from the washing machine spin cycle vibrating on the floor (Although I think it was likely the former!). We put the piping back together, silver-taped it to secure it and have used our vax hoover to drain the water from the solum -
Well.... safe to say we found the source of the smell alright!
My man spent the whole morning heaving under the house and I wasn't with him (being pregnant) I was standing outside the little door that leads under the house and even I was being sick with the smell. I can't even describe it - it was horrendous - it was by far the worst thing I've ever smelled and I have smelled some horrors let me tell you!
However, having an answer has been a total weight off my mind. I am so glad I just didn't assume it was the septic tank now otherwise who knows what would have been the eventual outcome!
I wanted to just say thanks to everyone that posted up ideas and advice because I really, really appreciate the expertise and the time taken to helping us out! I would never have even looked where I did if it wasn't for you guys!
Hopefully the problem is solved.