smell in bathroom

[NS215]

After some plumbing/drainage advice peeps...got in today and we get a strong sewage smell in the bathroom and as soon as you flush the toilet it disappears for 30 mins..only to come back again..too dark to check the main drain but everything appears to be ok..Is it a coincidence it only started since I got my loft lagged..and it appears they have lagged and sealed my header tank on Wednesday..
Any assistance most appreciated..cant even blame this on the dog..

Thanks in advance for any advice

 

[Burnerman]

Has the soil vent pipe been disturbed in the loft?
John

 

[Madrab]

Seen some really fabulous workers terminate stacks in the attic, not really a sackable but never the best but seen some without AAV's fitted. Wasn't pretty, see if this is the case for you.