people who were children during the war (IE three of my four grandparents) tend to be really nostalgic about this, I want to know if it actually made much difference or was basically just a way of getting normal people to feel more helpful then they really were?
I've had a sniff around and there isn't very much information on this, for reasons I'll suggest at the end, but:
The Telegraph reports the Royal Horticultural Society (I can't find the original source) saying that (I think) the allotments produced 1.3 million tonnes of food during the war, while stating that food imports in 1941 were 14.65 million tonnes, albeit that that was half of what we were importing in the 1930s. Even if we read that figure as being yearly (it isn't clear what the Telegraph actually meant to write), it's clear that the actual effect the Dig for Victory campaign was having on food shortages was small as it did very little to affect the UK's reliance on imports.
As you'll know, many foods (along with clothing, petrol and other things) were rationed in the UK from the start of the war until, in some cases, the mid-50s. As such, the government had a central control over products that were in shortage (meat, butter and so on). I think it's telling that there weren't any such controls on vegetables.
By 1942, the Ministry for Food was bolstered by large amounts of food being imported through Lend-Lease from the USA. The MoF also subsidised many foodstuffs mostly consumed by the poor (wheat, oats, bread, meat, potatoes and eggs) to the tune of £145m a year. Similarly, no government subsidy for vegetables.
To me this suggests that there wasn't a major shortage of vegetables at any point during the war, which may be due to the Dig for Victory campaigns. But I think it would be hard to tell, as by the very nature of the DfV campaign it was small-scale production at the local level with no-one tracking it.
You might be interested in a past post on the topic: