I'd like to try growing some butternut squash, is it worth trying to grow them from seeds from a supermarket squash, or better to buy seeds? And any other advice?

You need to buy seed if you expect any more than a long green plant! They will grow from supermarket ones but probaby wont fruit.
I bought some seed and not one of them has come up!

I planted 6 seeds last weekend and was unsure if i was getting to impatient as of last night nothing had started to push through. this morning i woke to find one of them had finally come through the soil and by this afternoon another 4 had too!!! this is first time of growing veg but so far (touch wood) everything has spouted. good luck

I'd like to try growing some butternut squash, is it worth trying to grow them from seeds from a supermarket squash, or better to buy seeds? And any other advice?

Where abouts are you, I have some Squash seed you can have that I got from that BBC Dig in thing last year, there are no dates on the pack, it's an unopened though and should still be good, I planted the rest of last years Dig in seed this year and they're all shooting up.