Thanks Guys I had a very quick play with the JP Diag software and a cheap Amazon Mic last night. It appears to work incredibly well, so when I've got more than just the 10mins I had last night I'll be using that. The issue I've had with several different Apps on my mobile appears to be simply down to my phone mic not reacting quick enough to pick up a short sound. Nasher.
Bottom A on a bass guitar is actually 55Hz. Bottom A on a guitar is exactly the frequency you want, ie 110Hz, so a good guitar tuner should do the job perfectly. There are loads of guitar tuner apps for both iPhone and Android which will do very well.
You can still do it with a tuner app, the good ones allow you to set a reference frequency. If you set concert A to 400Hz rather than the "standard" 440Hz, when the belt tension is correct it will show as tuned to A
