Skipton BS issued these bonds in March 1992, with a nominal value of £1000. each. These are not covered by the Financial Services Compensation Scheme, which goes someway to explain the very generous interest rate.
Hargreaves Lansdown are currently indicating a mid-price of £210, which would be for a nominal £100. giving a nominal return of 6.131%. With current interest rates being virtually zilch, this investment would seem to be very attractive.
https://www.hl.co.uk/shares/shares-search-results/s/skipton-building-society-12.875-pibs
Not having the tools nowadays, I cannot check to see if these bonds were issued at a premium, it all depends on interest rates and expected returns at that time.
I am no longer involved in the Investment business and therefore not authorised to give investment advice, therefore if you have some of these bonds or wish to buy some, you need to speak to an Authorised IFA or a Stockbroker.