1.Treat as SR and really really hope to lose at the bookie for the full free bet.
2.Or treat it as SNR for a guaranteed profit but hope to win the bet.
3.Or underlay so that your lay risk is £25 less than it should be for a guaranteed minimum 50% return on the offer but hope to lose at the bookie.
Back and lay odds of 5 (assuming 5% commission at betfair)
Seeing as a £50 back bet will net you £200 profit:
1.A SR lay stake of 50.51 gives you a lay risk/liability of 202.02. So a loss of 2.02 either way but you might get a free bet if your lay wins.
2.Treat it as SNR (it's still SR in reality) and your lay stake of 40.40 gives a liability of 161.62.
If the bookie bet wins you're now 200 up from that and 161.62 down at betfair - £38.38 up overall
If the bookie bet loses you're now 50 down there but 40.40 up at betfair - overall a loss of 11.62 BUT you now have the £50 free bet. Make 40 from this and you'll be £28.38 up overall.
3. A bigger underlay of 43.75 lay stake gives a liability of 175.
Bookie bet wins =200 up there, 175 down at betfair= £25 up overall
If the bookie bet loses you're 50 down there 43.75 up at betfair = 6.25 loss but with the 50 free bet to come. Make 40 from that and you're now £33.75 up overall.
The above illustrates that when you start changing the underlay amounts your profit levels are dependent upon the results of the event and seeing as you cant decide whether to win at the bookie or not - you pays your money and makes your choice. Either way though, unless you go for option 1, you still cant lose.