I read your article with great interest, as I'm also around the same age and building towards a "grown up" portfolio after a couple of years of dabbling in the market with mixed success (under no illusions as to my ability as a super investor with a couple of reasonable gains followed by several losses!).
I'm currently holding 6 companies (TSCO,AZN,REC,SSE,AV,GSK)and am ultimately aiming for 10-15, with BP,VOD,RB and ULVR on the current watchlist.
I'm a little biased on the pharma side at present with around 40% of my total holdings in AZN and GSK, and I'll be looking to balance this out a bit with future purchases, but I felt they were both good prices to get in at, and with both being heavyweights with a strong dividend I'm not really losing any sleep.
I've now settled into a comfy regime of monthly additions to my ISA (with the odd top up from time to time when I can afford), and one or two purchases per year, so it'll take a bit of time yet to get to my ideal scenario of a low maintenance portfolio which favours high yielding stocks, but it's a comfortable strategy which works for me, as I now don't miss what I'm putting away each month.
I recently did a simple projection of where I'd be with my investments in 25 years, (assuming a continous drip of cash each month and measuring against varying degrees of success) and found it really helped to focus my mind on how I need to shape my investing strategy. I've decided to monitor my portfolio performance against my projections with a quick (20min) review each month for the next couple of years to see how I'm doing, and may well change according to progress, although the hope is that I won't need to do much!
I personally like the stocks you're contemplating, so I would agree with your top-up strategy at present - but I also agree with some of the comments above and I'd take some of the gains you've made to further top-up some of the bigger companies at great prices!
Good luck however you decide!