A search of the database for
capita
reveals one entry with a Colchester number in it:
Capita Insurance Services 0870 523 4500 01206 757500 Colchester
This is verified. Note the similarity between the 0870 number and those supplied in the OP; only the last two digits are different.
Indeed, note that the last three digits of 0870 and 01206 numbers are the same. This could be a coincidence or it could be indication of a pattern. For example, perhaps 0870 523 4567 is mapped to 01206 757567. If this isn't the case, then maybe 757500 is the main switchboard (as it ends 00) and will put you through.