You're so close to me in your investment career I felt the need to join up and comment. You're basically where I'd like to be in about 6 months having started investing in a similar way about a year ago.
I think you've already had some pretty good advice. Even if you doubled your positions in any one company from MITIE down then it wouldn't look out of place (you've already ruled out everything above that for good reasons). I also like the idea of you picking up a big oil company. I personally hold Shell and think that their extra R and D spend, solid value credentials and the fact that everyone is talking about BP puts them ahead. However, Beyond this I don't think there's a need for you to look for another big company 'forever' type stock, you've already got around 6 out of 15. I would suggest that adding to the positions of those you have (I would suggest LAND and VOD, I hold neither but having been looking at both for quite a while) would reduce the risk of your portfolio becoming a FTSE 100 tracker (research has suggested that 15 companies is plenty to form a tracker for a private investor). That said, I work in the power industry and out of the 'big 6', I think that SSE are the best out there (I don't work for them nor hold any shares).
I'm definitely a 'value' guy but that doesn't mean you can't go in for some small caps too. I would suggest (with no meaningful research) that IMG's value has been realised (either completely or in part) and you should look at the company's prospects anew and ask yourself if you would buy them now, at the current price. If not, seriously consider taking your profits and finding a couple of other small cap prospects (which can still have some serious value credentials).