I prefer (a) since it avoids any assumption being made about what a "call out" is - someone might argue that they did not call you out, but booked in advance. Whereas a "minimum fee" is less ambiguous.
A little also depends on how you charge the part hours. Say you do a job that actually only takes 5 mins on site, you could price it at £45 on the basis that is the minimum invoice, or you could say £25 + £10 - the first bit covering your overhead cost of getting there, the second bit being your smallest unit of chargeable time - assuming it was half an hour at £20/hour.
On the subject of rates, you have been charging the same for some time now - perhaps an increase would be in order - at least in line with inflation. `