Is it an amicable split? Are you on good speaking terms?
In theory, I think she's only entitled to half the equity increase since the date she moved in.
i.e. if it was worth �100k when she moved in, and you had �80k left on the mortgage, and it's now worth �130k with �70k left on on the mortgage, the equity has increased from �20k to �60k, so a �40k increase, in which case she'd be entitled to �20k.
If you can prove she hasn't contributed to anything (bills, food, mortgage), then you might be able to recalculate this value taking off the amount that she should have paid (half mortgage, food, elec, gas, counciltax etc... over X years since she moved in)