With the bit about IANAL ...
If it was not made clear to you in advance that they'd try this chargeback on leaving, then it's not legally enforceable because it did not form part of any contract you made with your employer. Signing a piece of paper after the event, even if they did make you aware of the details, would still not make the term contractual - unless you explicitly agreed it as a variation of the original contract. Contract law is quite clear that the contract has the terms agreed at the point the contract was entered into and cannot (generally) be varied unilaterally by one party later.
As mentioned, if you say nothing, they will probably deduct it from your final pay. If you have contested that it's a valid deduction, then technically it would them become an unlawful deduction from wages - a specific law brought in to protect workers from unscrupulous employers who would tack on various made up costs in order to short-change employees. That is something you can go to an employment tribunal with, and it costs you nothing.
That, AIUI is the legal position. How you want to play it is up to you.
It is worth knowing that there is no cost going to the tribunal - the tribunal service tried adding costs, but that was struck down well and truly by the courts when challenged (IIRC by the unions standing up for their members). But, your employer should know that it will cost them a darn sight more than £170 dealing with the tribunal - even if they were to win. So perhaps a quiet conversation along the lines of "well actually, it's not something I agreed to when doing the training - so wouldn't that be an unlawful deduction that the ET takes a dim view of ?" Any manager with a few brain cells switched on should be able to decipher that and realise it will be a lot less hassle to "waive" the reimbursement.
As an aside, at a company I worked at many years ago, one of the directors told us over dinner about going to a tribunal. They'd employed an accountant, and an unsuccessful candidate took them to the ET claiming sex discrimination. So three senior managers/directors had to take a day out of the office, travel to the other end of the county, and defend the claim - which they did rather easily. The claimant said that they'd employed someone less qualified for the job simply because he was a man - and that was sex discrimination. "So", asked the panel (made up of three women), "why do you think you are more qualified than the person they did employ ?" His response was "Because she's a woman". Case dismissed, but it still cost them to defend the claim.
It's tempting to think the claimant should have to pay costs in such vexatious claims - but that then creates a financial disincentive for people (especially the lowest laid) to stand up for their rights, which was why charging for using the ET was ruled unlawful.
As for company finances/tax. The employer will have (or will depending on timing) set the cost against profits when calculating their corporation tax. So (IIRC) they'll save something like 18% of the cost of the training. Also, they'll have reclaimed the VAT element at the end of that quarter - I'm assuming they are VAT registered.
If they do charge you for the training when you leave, both of those should get reversed - though TBH I'm not sure whether such a charge would be subject to VAT or not. Essentially they should end up in the same position profits/tax wise as if they'd not paid for the training.
EDIT: Doh, and then I see it's a resurrected old thread !
