It seems to be a tenner for me at the moment. If they do the offer every week then I'll be more than happy to take a £10 bet for nothing on the correct score.
If you have £20 on the over 0.5 goals then the best option is to have another bet somewhere to cover the 0:0. This now sets you up ready for a matching no-risk £20 in-play bet on whatever you fancy.
The key to the Bet365 offer is that they don't restrict the odds of your bet i.e 1/2. This means that you can have a virtually risk-free £50 on the over 0.5 goals and either go for the in-play draw or cover the 0:0 and go for something more exotic for your in-play bet.