Carpet on Allotment. Is it safe?

I doubt there's anything to be concerned about, but if you're worried you could plant a crop of green manure and see how it turns out. If it grows healthily then it's probably ok. I'm not sure what chemicals would leach out of a carpet that would be dangerous. The carpet itself will be either natural fibres which will rot, or synthetics which are oil based hydrocarbons and will just breakdown into smaller pieces (like the underlay someone left on my plot). The worry would be if they've been treated with chemicals, but since they're used in homes you'd expect them to be mostly safe. On balance I'd be more worried about the chemicals that previous plot owners had used or what had been put on bonfires on the plot as this can generate some nasty chemicals such as dioxins.

what you don't want to do is get rid of it at your expense so  tell the council/committee you got the plot from that it can be put into a pile for them to remove.It cannot be burnt.

I'm in the "worry about it" camp.

To me its like using tanalised boards for raised beds.  The boards are treated with chemicals that have been tested and found to be "safe" but presumably assumed to be used in dry construction, not half burred in wet soil in close proximity to root vegetables that will be eaten?

Conversely chemicals used in agriculture are tested in the environment in which they are used before being pronounced "safe" (and many/most? of those still get banned because they prove to be dangerous after a decade or two of being the "Best thing since sliced bread")

I feel the same with plasticisers on synthetic carpets (or synthetic backed carpets).  I expect that Best Witon is fine - all natural materials.  Probably rather rare on an allotment unless the previous owner was refitting the Queen Mary!

Weeds growing through carpet, as it starts to rot down, are a nightmare too - makes the carpet very hard to remove, hopefully you don't have that problem.

If it enough to worry about? Dunno. There will have been lots of previous owners of your allotment patch, some may have used creosote and Jeyes Fluid to control pests, a while back probably DDT and the like too ... I expect the soil has recovered since.

I put Glyphosate on mine, initially, for a kick start.  Its had no pesticides / herbicides since, but I still worry about using Mypex for mulching, tanalised boards for raised beds, and so on ...

Carpet is banned here, unless it was there beforehand (rule came in this year).  Our council will take it away for you at huge expense to you.....

I have been known to use it on paths, I wouldn't use it on beds 

I have been known to dig carpet out from under what were six foot weeds when clearing a plot and roll it up to go in the pile for the skip. I have been heard to say a number of pertinent rude words on the matter as it was large and heavy to move. The other allotment holders understood the gist and nodded in agreement that it was a thoroughly bad idea to use carpet. It was a very visible demonstration of a bad idea.

My wife loves the white shagpile carpet I have had fitted in the shed. Seriously though alot of diggers here use it on the footpaths cos it keeps their wellies clean . I have been finding bits of the stuff all over my plot so just ripped it up and tipped it.