The only way to be sure of Martha's maiden name is to order one of the children's birth certs - there is a marriage for Thomas Morgan and Martha Probert in Knighton registration district in jan-mar 1845. There is a Martha Probert in Old Radnor, aged 26, on the 1841 census.

To order the cert, go into the website I gave you earlier. You have to register, which doesn't cost anything, and you'll be given a password, which you can use for later orders.

You will be directed to a page, Certificate choice.
Scroll down to Birth cert. Click on the button.
Scroll down to 'is GRO index known' - click on button 'yes'
Enter the year of one of the childrens' registration (1852 for Frances or 1859 for Jane Ann)

Click on 'submit'
Next page - enter your address etc.
Submit.

Next page - enter details of Frances (or Jane) - her surname and first name, place of birth (Bedwellty) and father's surname (Morgan)
Scroll down page and enter quarter of registration, registration district (Abergavennny) vol number and page number (which I've posted earlier)
Scroll down to delivery - standard.
Number of certs -1
Don't worry about personal reference
Checking points - you can use this to make sure the cert you are ordering is for the right child. If you use her parents' first names (Thomas & Martha), if the cert is wrong, you won't be charged the full amount.
Submit.

Next page is 'basket summary' - gives you the chance to change if info is incorrect

Next page - proceed with payment - click on that.
Next page - enter your method of payment- debit or credit card. And the order is complete.

Hope this helps.
Christine