Yes, there are lots of unaccredited former polytechnics which will do that. Use Google to find them: https://www.google.com/search?gcx=w&sourceid=chrome&ie=UTF-8&q=MBA+top-up+site%3Aac.uk

Compared to an MBA from a low-ranked school, a better alternative would be to use the PGDip as credit towards an MSc from a well-ranked university. In terms of well regarded schools, I'd start by asking Warwick, Henley and Ashridge what's possible. The Open University would be worth asking too.

For credit accumulation and transfer, a PGDip counts as 75% of an MSc. Each school may ask to see your transcript and add its own requirements. Obviously, a PGDip from Oxford is going to count considerably more than a PGDip from Portsmouth.

That's a good suggestion.

Thnx for suggestions for MSc..
But not a single accredit institute does top up MBA...?

No.

Duncan's right and for good reason: 'top up MBAs' aren't worth the paper on which they are written, IMO.

In contrast, check out the ranking and reputation of places like Ashridge and especially Warwick. (Both 'superbrands' in the thread on this subject.) If your PGDip is from a lower ranked school, then upgrading it to an MSc from Ashridge, Warwick, Henley or the Open University would be very valuable. E.g.
www.wbs.ac.uk/students/masters/
www.ashridge.org.uk/website/content.nsf/wDEG/MSc+in+Management?opendocument

In which management discipline is your PGDip and where did you study?

The top up will depend on the institution from which you have done your PG Dip. have you done from a private college or from a UK University campus