Don't know if this site will be of any help...

http://govt-files.com/uk-phone-trace.htm

The nearest matches I can find for these numbers are:

0870 1566485 - Barclays Bank Security Section, thread here

0870 8880225 - Next Directory, thread here

There is no guarantee that the numbers ringing you are the above companies. However, it is not uncommon for one company to have a block of consecutive numbers. The 0870 1566368 number is less than 100 numbers away from the one above and Barclays is a large company, so may have many 0870s, so if you do have a connection with Barclays then it could be them.

Not so sure about Next as the number is further away and they aren't likely to have as many numbers as Barclays.