Welcome James, but you should probably address this to the StackExchange legal group.
The answer depends on too many unknowns. Your best bet is to book an appointment with a local Citizens Advice Bureau and take all company paperwork with you, especially your contract of employment and any company policy documents (employee handbook etc).
Your immediate claimed liability (how much they say you owe), depends upon whether your company has a 'carry over policy' (can you carry unused holiday from one year to the next), how much leave you have taken this year, and when the holiday year starts (this will usually, but not always, be from January).
Accruing this past even 1 period (year) end does seem somewhat negligent, never mind 6 years. They should have raised this as an issue at the time.
Finally, they can't just deduct the money from your salary; you have to agree to it (which will mean explicitly signing something to that effect. "If you do nothing for n days, you're deemed to have accepted" will land them in serious trouble). If you do owe them some / all, your CAB advisor can negotiate a payment plan with them.