Struggling small businesses who are fighting to prove they were mis-sold interest rate "swaps" with their loans are to leaflet staff outside Clydesdale Bank headquarters in Glasgow this week, in the hope of unearthing whistleblowers.

The NAB Customer Support Group held its first Scottish meeting near Dundee yesterday and said it would appeal to bank employees to help its campaign. John Glare, spokesman for the group, said: "We are hoping there will be members of staff who are directly involved with tailored business loans who might be able to speak to us."

The group took legal advice on how to break the impasse which has seen the Clydesdale and other banks invalidate complaints relating to fixed rate loans – typically the Clydesdale's tailored business loan – despite the fact derivatives experts say they contain embedded interest rate swaps.

One property developer, who like others did not want to be named because of his dependency on the Clydesdale for continued funding, said he had been told he was not eligible for the current Financial Services Authority review of past sales as he had a simple fixed rate loan with no swap. "After five letters saying they would investigate, they said you are not covered by this but as a goodwill gesture we will offer you £200," he said.

The developer had, however, managed to obtain a statement from the bank that it had made a commission of £64,306 for selling the product.

Mr Glare said this was the first admission that so-called fixed rate loans earned bank salesmen the levels of commissions normally associated with interest rate swap agreements.

Clydesdale Bank has told The Herald: "A fixed rate loan is not a swap, and they should not be confused."

The developer went on to complain that the Clydesdale's withdrawal from commercial property now under way had left businesses trapped, yet burdened with breakage fee liabilities from hidden swaps. "It has made it financially impossible for us to rebank or to sell the properties that we would have done – we are backs against the wall," he explained.

Mr Glare told the meeting the bank had to be brought to account "for the damage they have done to our businesses and to jobs and the economy".

Clydesdale Bank says its strategic withdrawal will have no significant impact on the economy, adding it is "committed to maintaining constructive dialogue with customers", and breakage fees were always "fully explained".

Daniel Owen, youth pastor at Glasgow-based Destiny Church Trust, said the charity's social action programme in 500 churches was suffering because a £240,000 sale of flats had been aborted due to a £175,000 swap breakage fee levied by the Clydesdale. As The Herald revealed in August, the charity is suing the bank for £100,000 damages. The bank has said it has "acted professionally and fairly at all times" with Destiny.

Gordon Deane of Balfour & Manson said he had negotiated several out of court settlements with banks over mis-selling complaints, and he recommended exploring the use of "creative" funding solutions with the profession.