lower back ache

Another cause (from my personal experience & er'indoors) is glute's not firing, your large and small butt muscles should be one of the first muscle groups to 'fire' when taking a load. If this doesn't happen the body compensates by using another muscle group.If not rectified this can cause disc problems, for which er'doors has had three ops, one of which being a repair job (loss of spinal fluid causing the brain to sink!). So, it's most probably a bike fit problem but go and a visit a sports masseur/physio if you haven't done so already. They should be able to recommend stretches as well as strengthening exercises.
TTFN

Bike fit for sure,66yrs old,riding for 57yrs,never had lower back ache,but do sometimes get a bit of neck ache,especially on cold winter rides after 2/3 hrs.

I've just had a look at your profile, try going back to Condor during a quiet period and see if they will take a look at you on your bike on a turbo.

Get a fitting. If you've been riding for a while then it's quite possible that your saddle is too high or something else isn't quite right. Saddle too high is a common problem which results in your hips rocking slightly when you pedal leading to back pain. Search the web for Steve Hogg's bike fitting web site, there is loads of useful advice on there.

Definitely a bike fit, and try core exercises regularly. I had the same problems as you but both of the above have reduced my pain considerably. I comfortably ride 70 miles+ before I get the sort of pain I was getting previously at 50miles.

Try these:
http://www.bicycling.com/training-nutrition/training-fitness/core

I tilt my saddle forwards just a touch and i find it really helps. i have to fight off the spirit level brigade every time i post a pic of a bike i'm riding on here, but i can deal with that

Become more flexible. Tight Hamstrings and IT bands in particular can have a knock-on effect to lower back muscles. There's loads of exercises on t'interweb to loosen these up a little.

Bike fit.