Because you are purchasing a "new" recording of the track this comes under the jurisdiction of MCPS, the Mechanical Copyright Protection Society. So long as the track has been commercially recorded before, the process of "clearing" it for reworking/re-recording is usually a simple one and consists of applying to them for a license and paying a fee equivalent to 8% approx of the selling price of the recording. Thus if the work involved in producing your arrangement was £50 then an additional fee of £4.00 would be payable to MCPS. You can then rest assured that no one will challenge you for using that recording.

"Off the shelf" backing tracks will already have an MCPS licence.

Please note also that the venues used for festivals/competitions will normally have PRS and PPL licences for the public performance of recorded works.

Sandpiper will deal with MCPS matters on your behalf if they are relevant to your project.