.... my general PC for everyday use, but as a keen photographer I'm now spending a lot of time in Aperture and Photoshop.... I don't know whether a new iMac is going to give me much more performance, or whether these programs justify giving a current (or even recon) Mac Pro some consideration?
A couple of things.
The photographs and not so much the programs that are likely a tipping point issue. If your photos (and intermediate working files) keep getting larger that may be a case for a Mac Pro. That is indicative that the needs will change over time. More flexibilty would have higher value in that context.
If the photos and workfiles aren't much bigger and you just have alot more of them then it is not so clear. Storage on a modern iMac can scale, there isn't a huge gap there anymore.
Second you can do some measuring to see where your problems lie. If open activity monitor and then engage in a normal workload then can take a break in a session and take some glimpses at how much RAM using ( "Free" being a tiny sliver is not good ) and disk bandwidth ( single or low double digit MB/s ).
If I go the Mac Pro route, what spec do I need to give an appreciable benefit over a top line iMac on photo work ? I don't handle video at all.
Given the relative parity the 2012 iMacs (mid to upper end ) and the 2012 Mac Pro models ( entry level ) have on CPUs there isn't much differentiation to be found there.
RAM, disk I/O , and Screen are probably were there may/maynot be a gap.
The 2012 iMac screen should be better.
RAM depending upon how high you need to go could be able equal since these photos are probably not unusually large. High quality 3rd party RAM is a better bang-for-the-buck. (Apple's mark-up for 16GB is rather steep. That kind of extra money could buy a entry level screen calibrator or even AppleCare. You have actually have to buy the warranty before start worrying about Apple not delivering. Properly inserted quality 3rd party RAM doesn't have an impact on terms. If conservative keep the original RAM in case get into a "blame game". ).
Disk I/O. Simply going to two ( or more ) disk will probably help if seeing disk bottlenecks in your current set up. Fusion Drive or a deadicated SSD will probably work better. The Mac Pro doesn't necessarily win there.