Has anyone suffered with the same problem as me?
On the last couple of rides i've been suffering with an ache in my lower back. The pain starts after about 10-15 miles into the ride and the further I ride the worse it gets. At the same time I feel as though I have a groin strain. I don't think it's the set-up of my bike as i've ridden the same bike all summer without any problems. The only change i've made is putting on winter tyres which drag a little compared the my summer tyres.
Any help or advice would be greatly appreciated?

Yip. I have advice. Do rides that are 10-15 miles long and done worry about it.

I've had lower back pain for all of my adult life and, curiously, cycling seems to be one of the few things that don't make it hurt.

A few bits of advice:

1) Stretch, apparently.

2) Strengthen your back and abdominal muscles with exercises (to be found in many places on the web).

3) As your pain is fairly recent I'd get it seen to ASAP. I wish I'd pushed doctors into taking more notice of my pain rather than letting them send me away with a leaflet. Seriously, if it doesn't sort itself soon, get it sorted with medical help.

Good luck!

I've found that tilting your saddle to point slightly down helps.

as per bananaworld you'll need to do core strength muscle training exercises. get your gp to book a slot with the physio. they'll give a nice booklet too!

I suffer lower back pain if my bike is setup wrong, literally millimetres in it. Saddle too low, reach to short, bar height too high, tyre pressure too high etc etc results in back pain. Its taken me a long time in a trail and error process to get it right but now I literally set a bike up using a tape measure and I can ride for hours and hours on a hardtail without pain.

I had my pelvis straightened about 8 years ago after years of phsical labour and poor lifting technique and since have always suffered with standing for long periods but sport has never really affected it until now and it really annoying.