Advice needed if possible. My pace drops off slightly in mid-race.

I can start off ok, then in the middle 3rd of a race (e.g. after 2k of a 5k race), my pace will drop off a bit, then I'm able to pick it up towards the end.  Oddly enough, there were a few races last year when my last kilometre was my fastest.  Is it that I need to toughen up mentally when I feel my pace dropping and just dig in a bit more?  Would some sort of training geared towards negative splits help?

I dont think you are alone I believe the majority of runners slow down sligthly in the second half of any race regardless of the distance.  Ive managed to deal with it by pacing myself either with my garmin or with another runner - its tough but it has made a big difference because I simply dont allow myself to do what my body naturally wants to do which is slow slightly and I force myself to keep with that person who is my pacemaker or my what my garmin is demanding -   Hope that helps ! 

It could be that you need to dig in mentally. It's very easy to lose focus during a race. My last 10k was like that, I was running alone and just forgot that I was racing for a while. It happens most when the terrain is a bit uneventful. I like a few ups and downs to keep the interest.

Try talking yourself around the race, keeping up with the runner in front, or passing the runner in front if they are slowing down a little.

If you want to do any kind of training to help then try running at a steady speed, say your 10k pace for 3 mins, then speed up for 30 secs then drop back down to your 10k pace for 3 mins etc.
Another you can do is an out and back run. Run hard for 3 mins then rest for 90 secs - say jog for 45 secs turn around and jog for 45 secs back to where you finished your 3 min run then run hard for 3 mins maintaining the same pace that you ran out on. You should end up where you started.

These will help you maintain pace and adjust your pace if need be. Speeding up and slowing down is easy on undulating terrain but a lot harder on the flat. Try it.

During a race watch the mile marker or k markers and keep an eye on your timimng. Make sure that you are pushing yourself throughout the race. I talk to myself, I ask how much is it hurting, can I push a little more etc. Keep the narrative up, hard if your running for a long time and swtch off, I've done that so often but worth doing.

Thanks for that.  Camillia, I think I'll try something like that in a session.  I think you're both right on the pacing.  I must admit, there have been races in the past where I've been with someone of a similar ability and made sure I stayed with them.  I think I need to make a conscious effort to do that more often.  Good point about pushing myself through the race and if my split times start dropping off to dig in a bit. 

When I first got back into the running about 3 years ago, I'd start off at the back of the field, the once it opened up, I'd try and overtake people one by one.  It's probably easier to do that from a pyschological point of view.  Now I've got to a reasonable standard, I start much further up, usually near clubmates who are of similar ability.  Sometimes, it's down to a bit of tiredness on my part, but then I remember some wise old words that I once read:  "Remember, you don't get tired, you just get pleasantly weary."