rented a lovely house from managing agents about 3 months ago. moved in 4 weeks after start of lease, and noticed awful sewer like smell immediately. not able to pinpoint the source of the smell, or do anything about it other than copious amounts of airfreshener, so called agents. no problems, they send plumber round. he has

a look, and thinks it might be drainage from bathroom, which is enclosed as it passes thru sun room. he needs owners permission to do this, so back to letting agent.
7 weeks on, and numerous phone calls later, and i'm no further foward. i have written and asked very politely for them to do something, no response after 4 days.
what can i do? i'm wondering if i can inform them i'm getting quotes from plumbers and will be taking one of the quotes up to do the job. i think i have been very patient, but this smell is affecting my quality of life now.
any advise please?