Based on my understanding (I could be wrong, so apologies if I'm corrected), a Diploma of HE is usually awarded for 240 credits, essentially Y1 and Y2 of a degree course. It looks like you didn't garner enough module credits to warrant neither an honours or ordinary degree but have enough to graduate with a lesser qualification (the DipHE).
I would have thought your results would state whether or not you have been given a referral (resit opportunity) for those credits. Probably best to clarify with the university, otherwise there is always the OU, as Jenx301 has suggested.