Rigby and Fairy. The reason I mentioned details already being entered on tax returns is that I think the intention is, when Universal Credit is finally rolled out, to integrate the tax and benefit systems. In theory, that means that people with irregular income can claim credits when their income goes below a certain amount and will avoid unnecessary paperwork, because the information is already available in real time.
I don't think that HMRC and the DWP's computer systems are 'talking to each other' yet. Other agencies, such as local councils and the NHS, are also involved in administrating certain benefits. However, in the future they will. This should reduce opportunities for fraud, but I guess some people will find their way round it all.
PS. Rigby I thought it was a £1 deduction for each £250 of savings (but could be wrong - I'll check). I wouldn't wish living entirely on benefits on anybody. It's grim, especially for working age people, for whom the thresholds are much lower.