While most of us celebrated the glorious weather at the weekend, it was greeted with less enthusiasm in neighbourhoods around Isleworth.

With the sun came the all-too-familiar odour from Mogden Sewage Treatment Works, they said: cause to lock yourself in behind shut windows rather than join the national migration to the nearest patch of grass.

Members of Mogden Residents Action Group (MRAG) said the stench was back with a vengeance this weekend and they fear it is a harbinger of what's in store this summer.

The group's secretary Steve Taylor told getwestlondon: "There was a definite smell on Sunday evening. When my daughter came over for a barbecue, she said 'oh god, nothing's changed'.

"Although the smell wasn't that bad at the weekend, we're worried that if it's like this with the first bit of warm weather we have, what's it going to be like at the height of summer?"

Mr Taylor accused Thames Water of failing to efficiently monitor and address odour levels at the site, and Hounslow Council of failing to hold the water supplier to account.

Among MRAG's list of complaints is the fact Thames Water has not posted readings from odour monitors around the site since January this year and has yet to install an additional monitor near Bankside Close, as the group says was promised.

"Thames Water is not doing what it has promised to do and the council is too wet to hold it to account," said Mr Taylor.

A spokeswoman for Thames Water said: “We work very hard to monitor and report odour levels at Mogden. However. the publication of our latest report has been delayed because we've had problems downloading the data. We've now fixed this and are focused on getting the information up on our website by the weekend."

She added that it had noted no odour at the site over the weekend, though it did receive one complaint from a resident of Worton Road.

She also said monitors tend to last only six months before requiring repair, which takes six to eight weeks, which is why they are often out of use.

As for a new monitor in Bankside Close, she said it was not considered necessary as the storm tanks were not used frequently and Thames Water followed a strict storm tank management plan to ensure odour was minimised.

MRAG won a major payout from the site's operator Thames Water in 2011 , the full scale of which is subject to a confidentiality clause. Following a lengthy legal battle, a high court judge ruled the utility firm had failed in its duty to tackle the pong.

Thames Water has since completed a £140 million upgrade of the site , increasing the capacity by half. As well as reducing the amount of foul water released into the Thames, the operator said the work would be 'odour neutral' and should help stem the smell emanating from its tanks.

Opinion is divided about how successful the upgrade, completed in spring 2013, has been.

Some people argue the smell is less noticeable, and Thames Water says it has significantly reduced discharges into the river.

However, others have said it was worse than ever last summer. The high number of complaints prompted Hounslow Council to reinstate its 24-hour complaints hotline and MRAG to threaten further legal action.

Teresa Qureshi, who has lived in Worton Road, Isleworth, for a decade, said the smell over the weekend just gone was 'horrendous'.

"We wanted to do some gardening but the smell was so bad we couldn't go outside. It's always worse at weekends," she added.

"Since the capacity was increased the smell's been the same as before, if not worse, and Thames Water doesn't seem to be doing anything about it."

In December last year, Hounslow Council announced Thames Water was installing new sensors on machines at the site and testing new technology to prevent sewage foaming up and creating a stench.

The work is due to be completed at the end of April this year but with complaints continuing the jury remains out as to whether it will make a difference.