Hi PhilBarnes, welcome to the site. Between us here you'll definitely find out why this year has been such a disaster for you, so please do read all of the posts to see if anything rings true with you.
Madcat is right to ask what part of the country you are in. Not only does it help us to determine your weather patterning, it might also have a bearing on your soil type.
As others have suggested, your set-up will matter too, as will the conditions on tour plot.
For my two-penneth; genuinely, how much effort have you put in? Please don't be offended by my question - a lot of newbies genuinely don't know how quickly weeds can grow and out- compete the veg but if you don't visit the plot for at least an hour every week how can you know what's going on?
Secondly, have you dug down to remove the weeds that have (excuse the pun) taken root? Having taken on four different plots I know that it takes a long time (3 months in the early season ) of committed digging and hoeing to get to a point where the weeds are knocked back enough to get some crops going. It's tempting to clear the weeds, sow some seeds then not come back for 6 weeks, but as a newbie how can you tell which are weeds and which are plants? You need to be prepared to keep coming back to check how things are getting on.
Often , only when you have gone through the hardship of failure year after year will you realise what you are doing right and wrong. Personally, I'd use the winter months to read, read, read the forums and seed catalogues (and John's book, vegetable gardening month by month amongst others) make sure you get stuck in in the spring. There's an old adage which might help you: never let a weed see a Sunday. In other words make sure you get down to the allotment to at least hoe off the weeds once a week and then you'll find it so much rewarding.