Did British intelligence spy on Royal Society for the Protection of Birds?

From ANIMAL PEOPLE,  April 2001:

LONDON–BBC television journalist Julian Pettifer recently received an anonymous letter advising him that he and five other people linked with the Royal Society for the Protection of Birds had been investigated by MI5,  the top British military intelligence service–and the findings had been shared with a senior executive of the Tesco supermarket chain,  reported Nick Fielding of the London Times on March 11.

Pettifer was formerly president of the RSPB,  chartered by Queen Victoria and claiming more than a million active members,  but resigned in July 2000 when a BBC expose of pollution risks associated with salmon farming caused salmon farmers to demand that Tesco halt sponsorship of a habitat preservation plan for the fast-declining skylark.

Tesco contributed about $480,000 to the project,  1997-2000, but is no longer involved.

Both Tesco and MI5 denied spying on Pettifer and the others–but both Pettifer and Fielding believe the information in the letter is authentic and that the source is a whistleblower within MI5.