My employer offered me free health cover to which i said yes. Now i find i have lost �700 a year tax allowance due to my employer giving me free [so called] health cover. anyone know if i can sue for this amount ?
If an employer provides any benefit to an employee (whether that be a company car, health cover, gym membership or whatever) on a notional 'free' basis, that benefit counts as part of the employee's remuneration and is therefore taxable. So you should expect to pay tax (either directly, or indirectly through a change to allowances) on that benefit.
You can't be taxed (either directly or indirectly) on what you've not received, so it seems logical that declining the health cover should restore any allowance. (Such matters are determined by HMRC, not by your employer).