I have used a Shure SM81 condenser, as well as an AT4041, with very good results on Dobros. Both of these mics do well on practically any stringed instrument when you want clean,transparent pick-up. The 57 tends to be a bit "peaky" in the mids on resonators, but that may be OK for some players. An Audix i5 makes a good alternative to that, with a more extended top end. Here in the States, the i5 is only about $10 (5pounds) more than a 57. I have used the i5 on several bluegrass and ethnic stringed instruments with great results.
You have to play with the placement because you have players who use slide noise to their advantage and some who want to keep it to a minimum. Basically, I start off pointing it at a spot that is halfway between the "biscuit" and the outer edge of the resonator. As you tilt the mic away from the bridge and more towards the neck, you'll hear how the slide noise changes. Play with that.

I seem to have the best success placing the microphone about 10" - 12" above my right hand. You can experiment, but depending on the situation you can alter the tone of resonator guitars (dare I call them dobro's?) significantly by placing the mic a little closer to the bass ports (screen holes or F holes) and working back and forth until you find the best tone.

IMO it's tough to beat a Shure KSM32 for live sound on a resonator guitar! Not that I've tried every microphone under the sun, but I have over the years played with various mics including SM57, SM58, SM81, KSM37, AKG C1000, AKG C535EB, and a few others that I can't recall at the moment. I've seen comments from others indicating that large diaphram condensor mics are prone to feedback; but that's never been the case for me (of course I'm usually playing with other acoustic instruments, not a loud rock-n-roll band). Under ideal circumstances almost any mic will give good results and under less than ideal circumstances even the best mic will sound crappy. I've spent way too much money looking for the ideal live sound rig for my dobro. As of this moment, I am pretty happy with using a KSM32 microphone.

Hope that helps!

RobA

Thanks guys. AT4041 seems a good choice but Moonbaby excuse my ignorance but where exactly is the biscuit situated on the guitar i presume its the hole space between the main resonator grill and the neck

http://en.wikipedia.org/wiki/Image:Epiphone_Biscuit.jpg

or does it tend to differ on each guitar.

http://en.wikipedia.org/wiki/Image:Steel_guitar-KayEss.1.jpeg

Would this position of mic placement ("pointing it at a spot that is halfway between the "biscuit" and the outer edge of the resonator") differ for the lap players of this instrument and would it still be efficient.

Thanks

Resonator guitar re

Thanks guys. At4041 seems a good choice but Moonbaby excuse my ignorance but where exactly is the biscuit situated on the guitar i presume its the hole space between the main resonator grill and the neck

http://en.wikipedia.org/wiki/Image:Epiphone_Biscuit.jpg

or does it tend to differ on each guitar.

http://en.wikipedia.org/wiki/Image:Steel_guitar-KayEss.1.jpeg

Would this position of mic placement ("pointing it at a spot that is halfway between the "biscuit" and the outer edge of the resonator") differ for the lap players of this instrument and would it still be efficient.

Thanks