The DWP has set up Benefit Integrity Centres to check on the validity of benefit claims (the integrity in question is yours, not theirs!).
There is an article providing information about the scheme here http://www.dwp.gov.uk/docs/touchbase-ezine-november-2011.pdf but it is very hard to read which, frankly, is inexcusable. I suggest copying the text, without the formatting, and pasting it into your word processor.
The benefits in question, currently, are Income Support (IS), Jobseeker’s Allowance (JSA), Employment and Support Allowance (ESA) and Incapacity Benefit (IB). Why the latter, when it’s being phased out in favour of ESA, I have no idea.
Personally, though, I won’t talk to them – I’m far too ill and I’ll not remember what I said, what they said, or even be able to answer reliably. Buggers can write to me. This might explain all the number withheld calls I’ve been ignoring lately, though.
I would advise you to do the same, and not answer questions of the top of your head when they call out of the blue – it’s far too easy, in that sort of situation, to omit a vital fact and, in many cases, quite impossible to provide the necessary in-depth information that can be provided in writing.
Alternatively, you could, I suppose (again, I wouldn’t), ask them to call back in a couple of days when you have assembled all your documentation and information
Times have changed, and in the past I would have said don’t treat these people as enemies, they’re just doing a crappy job. Now, though, with the dice so heavily loaded against the chronically sick and disabled, keep in mind that these people are not your friends; they are not, most assuredly, on your side.
So if they do phone, I strongly recommend that you tell them, as I intend to do – mainly because it’s perfectly true – that I am in no condition to reliably give information over the phone, for health reasons, and to please send me a form. Be polite about it – but be firm. Because an omitted nugget of information could cost you dearly.
(Hat tip to @narco_sam, Twitter)
Ron's Rants... 
Ron (and @narco_sam) thank you for this as a general rule I deal by post rather than telephone – will mentally add this to the list of nuisance callers such as the Windows virus scam and the sellers of PPI claims.