What did you think of it Tim?
Sorry Paul, I've been a bit busy and only got round to trying it today.
I created a gpx using RidewithGPS for a 16km/10 mile route through the lanes. Dropped that onto my phone by putting it in a dropbox folder, clicked on it and was given the choice of opening it with OSMAnd (which I put on my phone some months back) or Bike GPX. I chose the latter (duh). After minimal faffing I was presented with a Google map overlain with a red line showing my route, markers at 5km intervals and a blue dot showing my position. (there are other ways of getting it to load a gpx file described on the help screen)
I tend to ride with my phone in my pocket, so selected the "alarm when off course" and started my journey. It seems to poll position around every 15 - 20 seconds, so going deliberately off course didn't alarm (beeping noise in my headphones) until a bit after I'd gone astray. Getting back on course gave another (different) beep. It did pretty much what it said it would, although it doesn't seem to work when running in the background. It's designed to be used displaying constantly, with the phone on a handlebar mount, which isn't what I do, so I guess this isn't a very fair trial.
I used it on the same route going home and it didn't complain I was going the "wrong" way. Whether that's a good thing or a bad thing I'm not sure. It was a good thing for me, as I followed the line home, but I guess could cause confusion if on a circular route.
It doesn't do turn by turn instructions, but doesn't claim to either. I'm not sure whether it downloads the Google map when loaded with the route, or whether it gets more bits of Google map as the journey progresses. The latter would have an impact on data useage.
Overall, yes, shows position in relation to route, warns when you go off route and is simple to use.