Tony Buckingham, the British former soldier of fortune who heads Heritage Oil, the exploration firm, appears to have sought assistance from a would-be Conservative MP to get a foothold in Libya, following the role of UK forces there in installing the new regime.

According to leaked correspondence, Buckingham – with another ex-special forces soldier, former SAS commander Major General John Holmes – is working with a would-be Tory MP, property developer Christian Sweeting.

Buckingham is also buying into local Libyan oil operators, paying $19m last month for a share of a little-known Benghazi-based oil trader called Sahara Oil Services Holdings.

Sweeting wrote to William Hague this year on Heritage's behalf. His letter gives an insight into the world of political influence, in which Sweeting sought, apparently unsuccessfully, to get a meeting for Buckingham with the foreign secretary.

Buckingham himself had made sizeable personal donations to the 2010 Tory election campaign – £50,000 to Central Office and £5,000 to the key marginal of Carmarthen West. The local party chairman there was the lobbyist Stephen Crouch.

Crouch's own lobbying activities have generated recent controversy. He was revealed to have donated to an aide to the former defence secretary Liam Fox, and subsequently gained a meeting with arms sales minister Gerald Howarth.

Sweeting, who narrowly failed to win a 2010 general election seat in Torquay, wrote his letter to the foreign secretary on 10 May, marking it confidential and reminding him that the two men had met recently at the Carlton Club, a Conservative party watering-hole.

He said Heritage Oil was "untainted" by any association with the Gaddafi regime and "the UK should capitalise on this". He wrote: "The executive directors of Heritage would welcome an opportunity to meet you or your officials to discuss their proposals, to demonstrate that they are being provided in the national interest."

Sweeting, whose Bentley is said to be a familiar sight in Conservative circles, asked a favour: that UK visas be granted to the group of Libyan insurgents with whom he had been negotiating. "I would be most grateful if in these special and to some extent unique circumstances your office could ask the UK Border Agency to expedite the issue of a single visit visa to the Libyans nominated below … This visit would be hosted by Heritage."

Sweeting signed his letter "With warmest regards… Christian Sweeting KCSG MRICS". These initials denote his membership of the royal institution of chartered surveyors, and his papal knighthood in the order of St Gregory.

There is no evidence the four Libyans concerned received any special official treatment as a result. Foreign office sources say they were processed in the normal way.

Sweeting has set up a company, International Mineral Resources, with Patrick Newman. Newman, who has links with the British-Uzbek Society, also makes commercial introductions, according to Craig Murray, former ambassador to Uzbekistan .

Sweeting went to Benghazi to promote Heritage, with Gen Holmes. Like Buckingham, Holmes moved from the army to the world of private military companies. He joined Erinys, which had security contracts in Iraq, and recently set up his own firm, Titon. Holmes succeeded in "bumping into members of the UK military mission who he knew", on the trip, according to Sweeting's letter.

Sweeting told the Guardian: "I wrote to the foreign secretary at an early point in the Libyan rebellion not to lobby him but to inform him and ensure that our efforts were known to HMG.

"The request for visas was for members of the national transitional council and/or their key advisers – at a time prior to recognition when the only visa office available to those persons was located in Tripoli and consequently unavailable to 'rebels' in Benghazi."

The Jersey-registered Heritage Oil company declined to comment on the Sweeting letter.

At the age of 59, Buckingham himself has had a colourful history. He is reported to have served in the secretive Special Boat Service [the SBS].

His mansion at Bucklers Hard on the Beaulieu river in Hampshire, bought in 2004 for £4.5m, faces across the water to the former £5m estate of his ex-special forces colleague, Simon Mann, although Buckingham has not had contact with Mann since 2000.

Mann last month published his memoirs of the failed coup attempt in Equatorial Guinea which led to him spending five years in an African jail. His career began when he joined forces in the 1990s with Buckingham and the mercenary group Executive Outcomes to fight against rebels in Angola.

Buckingham parlayed Angolan and Sierra Leonian profits into diamond and oil ventures and built Heritage Oil into an aggressive exploration firm, specialising in winning concessions in high-risk areas, including Uganda, the Republic of Congo and Kurdistan. His fortune is estimated at about £500m.