It's surprisingly easy.
Your wife gets a job with NHS or in healthcare, my wife works in a care home for the elderly.
After six months; go to this site:https://www.gov.uk/apply-immigration-health-surcharge-refund, upload all payslips, give personal details and wait about 7 to 10 days. You'll get an email confirming or rejecting your application. If accepted, payment will be made. They say withing 6 weeks but my wife's have been paid in about 3 days.
Be sure to cover the entire 6 months that she is claiming for in the payslips. One day out and it will be rejected. We didn't know about this scheme until recently but have been able to make 3 successful claims, all backdated. We are now waiting until January to make the next one. Each refund is £200, this based on the old figures of £400 a year, before the rise in fees.
It is impossible to get the the full amount refunded. 4 claims would be possible and not 5. This due to the difference in time from visa issue to getting a job.