He's what you do.
You write to her via recorded delivery that you will give her 7 days to pay back the money. Failure to receive the money by then will result in you "having no option" to take her to the County Court. (Small claims court is part of County Court)
For a small outlay (£50-70) You can do it all ONLINE.
https://www.gov.uk/make-court-claim-for-money/overviewExplain in the letter that it's in her best interests as based on the "Tex message" you have, your "lawyer"

or CAB etc.. has advised that your claim is strong...and that judgement in your favour will affect her credit rating and ultimately cost her more than £500 (add CC Fee and your costs, travel etc..) + Bailiff
You can, if you win and she won't pay, enforce a:
1) warrant of execution
2) attachment of earnings order
3) third party debt order
4) charging order
Then, if she ignores you. File the claim. If she doesn't show up in Court, you win.
Don't be afraid, it's not like Rumpole of the Bailey. Just you, her and a guy in a suit around a table.
The worst that will happen is you lose the £50-70 court fee.The best is that you get your money back.
Trust me. When those official Court Summons appear on her door mat, its amazing how it focuses the mind. Probability is that she will pay up immediately.
Jonathan.