How is this figure arrived at?

I think it refers to principal recovered on previous year's bad loans during that period (on which you would have to pay tax if you had claimed bad debt tax relief on a previous year's tax return).

Relief for loans deemed irrecoverable under SAIM 12000 only came in from the 2016-17 tax year. Since most AC loans that were deemed irrecoverable that year have since been recovered, it's pretty much just a timing issue (ie. you paid the tax a year early)

The email you quote from April 2017 would have been superceded by AC's new tax statements which came in around Sept 2017 and included loans deemed irrecoverable and recoveries.

BTW, a loan deemed irrecoverable (ie. a defaulted loan that has entered legal recovery procedures) is NOT the same thing as a loss. Many posters on this forum cannot get their heads around this and repeatedly claim to have "lost money" on XYZ platform when all they really mean (at this stage) is that some of their loans are in default and legal recovery procedures have begun. Relatively few P2P loans in default have exhausted all routes for legal recovery and been written off as a loss. Across the platforms I've used in the last 4+ years, I've a couple on FC and about 6 on ReBS.

That said, there are a couple where I reckon AC should face facts and declare the remaining deficits as a loss (#57 Optical Lens Manufacturer and #86 Scottish Property Investment). Also a couple on FS where prospects for further recoveries also seem very low (Wind Turbine and Scottish Boatyard)