Hi Mick. Depends on how much you want to spend!. I suggest nothing smaller than a 105mm as your subjects won't be scared off by you getting close. There are plenty of used macro lenses from the likes of Mifsuds, Park Cameras, and others. I'm a Nikon user and got the Sigma 70mm f2.8, but I should have got the 105mm instead. Nikon macro lenses are expensive but do a great job, but I'm very pleased with the quality of my images using the Sigma which costs a lot less. Buying new is expensive but great lenses can be bought second hand, it's up to you. All the best.
Hi I agree with canismajor get the largest you can afford i have a 105 sigma that i use with a d5000. To be honest i find it a great lens ,not only for macro, but also for portraits ... good luck graham