The offer is: place a prematch bet on the specific event, then place an in-play bet on the same event. If your in-play bet loses, they will refund it up to the value of the prematch bet, max 50 units (pounds).
What bookmaker is that MM? Is it betfair?
There is a certain amount of risk with matched betting (which technically is covering all outcomes of an event to ensure certain profit thanks to the bonus money). Bookmakers frown upon such behaviour which is considered abuse of the bonus offers. In the T&C of the bookmaker there will almost certainly be a line that reads as follows or something to similar effect:
I would suggest always applying matched betting between two different bookmakers to ensure they don't void the bonus and/or profits.
In the mean time, this is looking good at the moment.