Thomas Higgins Ltd

This is another post of the type which almost defies belief.
Unless your former employer is trying to allege that you are somehow responsible for them making this overpayment, by fraud or otherwise, then the overpayment was due to their error.
Considering both the relatively low amount and the fact that it was their error, most employers would simply ignore it rather than openly admitting that their payroll department had made the error and incurring additional costs in attempting to recover the overpayment.
Their initial action in writing to you asking for you to repay the £28.88 is I suppose reasonable, although mean spirited, but their subsequent actions are not.
I am actually surprised that this firm of debt collectors would have even accepted your employer as a client to recover a £28.88 debt. Sounds loss making to me. How much are they charging your employer for this service? Even if they were getting 100% of the debt doubt that they would agree.
This is not debt incurred as a result of your defaulting a consumer credit agreement. It is a civil matter between you and your former employer. They need to contact their solicitors to arrange for legal action to be taken against you, not a debt collection agency. Of course as the cost of the solicitors time would far exceed the amount of the alleged debt, they wouldn't bother (assuming that your former employers are of sound mind). Or else they could take you to the small claims court themselves, but again the cost of staff time plus the effort involved would not be worth it.
As for bailiffs, nobody can instruct baliffs without a court order.so that is a very empty threat. They can do nothing to your credit report (unless a ccj is actually obtained which I am 99.99% sure never will be) as this is debt is not in respect of a consumer credit account.
Not sure why you only repaid the £28 and not the 88p, this seems a bit silly of you.
In the very unlikely event that your employer should proceed with legal action to recover the 88p, do they actually have evidence to produce to the court that this was actually an error on their part? Remember your employer will have to send a senior member of the payroll department staff to the court to stand up in open court and admit their error and produce acceptable evidence of the error. They will have to instruct legal representation, maybe even a barrister, to represent them unless they are a 'do it yourself' type of employer and the MD goes along himself. to represent the company.
If they produce the evidence and you don't defend yourself. then maybe, when he has finished laughing his head off, the judge may give them the ccj for 88p which you could promptly throw across the courtroom at them in 1p pieces! It is highly unlikely that they would be given an award for costs in such circumstances.
PS. Before paying anything, you will need a new payslip from your employer as you will have paid NI and tax on this overpayment which will have to be adjusted. Work out the amount, deduct the 88p from it, and counter sue them for the difference.