@dboy, the social contribution is not useless no matter if you are commuting or not, it pays your health care, your pension etc. you pay all that in Germany, but with that you will get health care in the country you live in and your pension contribution will be than taken into account and credited in any country of the EU, maybe even in a post Brexit UK, who knows.
But to answer your question, expect around 220-240h per month, that's about 750-800GBP but only about 50% of that is tax free.
@SG in your case I wouldn't apply at all.