Hey! All sorted!
So what happened was the £100 bank transfer action was made by my hubby who totally forgot he was playing around with the app so that solves that mystery.
With regards to why the £100 came out of my bank account but did not update the balance of the Monzo card was because during the bank transfer details you need to make sure you use the reference number displayed either in the app or on the bottom left of the Monzo card. This reference number was not added when filling in the bank transfer form. This is the only way Monzo can recognise the money transfer was from my bank account.
This was rectified by using the ‘help’ chat function in the Monzo app and Dillon very kindly gave me all the details of what happened. As the money came out of my bank but had no reference to who’s Monzo card it belonged to they had to do a manual transfer instead. So now my Monzo card has the correct balance.