NI qualification help

skintandhungry_2

hi newbie here. I've had the dreaded NI shortfall letter for 05-06. I contacted the nice people at inland rev, who after hearing my circumstances said that i needed to had earned upwards of about £3400 in order to have paid enough NI for that year.
I checked my payslips and i'd earned upwards of £8000 so i sent them in with a covering letter. I contacted them again to see what was going on and they said i was still short. I basically worked for 4 months before going on to SSP for 6 months then incapacity benefit for the rest of the year. They've said that the Incap benefit qualifies for contributions or deductions so that's ok. They kind of said the SSP doesn't automatically, so they said i should have filled out the form they sent me with the letter. I said i didn't have this so could i have another. They then said, anyway we've taken this into account and you're still short. So i explained about the previous phone call and my earnings of over £8000 in 4 months. They then said that it depends how and when you earn your money as to how it qualifies (or not) for NI contributions. They weren't able to explain it any better in a way that i could understand, so i'm trying to find out for myself how the earning and qualification works. can anyone help me or point me towards the right information?
Failing that i'll have to try my CAB. The inland rev may will be right, but i'm not paying up the shortfall without being absolutely certain for myself. They sounded more like debt collectors on the phone.
Appreciate any help thanks.