Are you operating through a limited company? If so, IT51 & IT55 apply and these don't force you to have receipts for every single expense item. If you're a sole trader, you must have such receipts.
I'm not sure if you're taking the same understanding as I am from what Strongback is describing, Tommy.
As far as I can tell, the auditor is looking at the company's payments of tax free travel & subsistence to the employee, and they are seeking verification from the employer to confirm that the employee was in fact away from their normal place of work on a sample of the dates in question. If you were performing a statutory audit of the company, is this any more than you would want to see to satisfy yourself that the claims paid were legitimate? It's very easy to have a claim logged which satisfies the information recording requirements of IT51/IT55, but the employer would need to be able to demonstrate how they satisfied themselves that the employee was in fact off-site during the periods claimed for.
For example the Revenue auditor will have claimed mileage and subsistence for the day(s) on site at the audit. They will have submitted a claim to their manager for the expenses, but if/when the C&AG's come to audit, they will look at the time clocking system to see that the employee was clocked out of the office at the time, and they might review the audit papers which should have the auditor's notes of interview etc on the date(s) in question.
The need to seek corroborative evidence like this has arisen because of auditors' experience in previous audits, whereby people have claimed T&S for driving around the countryside to clients, on dates when they were out of the country on their summer holidays with the family, or in hospital after an operation etc...
While in strictness Strongback's agent could probably tell the auditor to go whistle for receipts (since these would be the individual's rather than the company's), it is the easiest way to clarify the position, and if Strongback has them and wants to get the thing sorted then it'd be sensible to furnish them. If they don't want to play ball in that regard, then the auditor will need to see diaries, timesheets, invoices (don't engineers normally charge a per diem / mileage on the invoice for site visits?) or other correspondence with clients referencing the site visits... bottom line is they'll have to produce something to support the log.
It's also worth bearing in mind that the treatments in SP-IT/2/07, IT51 and IT55 have no legislative basis, they're entirely concessional.