Where to start my new plan?

hmmm...so basically you need to triple to quadruple your bike and about double your run (to go over the distance some) then just swim.  well, first...if you want to follow the 10% rule and take once a month as a 60% EZ week...get a peice of paper, calendar and calculator:

1.  just take your CURRENT long run and bike and swim times now, and each week multiply them by 1.1 for weeks 1, 2 and 3 to get the next weeks long time...this will correspind to 10% increase in volume from the prior week.
2. for the fourth week, just take the third week and multiply it by 0.6 to get 60% of the volume (ez week)
3. now for the short sessions each week for 1, 2, 3 and 4, multiply by 0.6 of the long session to get the 60% of the long session....for mediums (should u choose) the long run would be multiplied by 0.8 for 80% of long volume.

you are basically going from a sprint to a 1/2 IM for this year...careful.  if you cant make (optimally go over) the distances by the above general rules...u could be in for trouble...listen to your body...build in a 'maintenance month' where u just maintain volume...work in some diff stuff so u dont get bored.  BUT u may not have time for a maintenence month.  BUT only you know what your capable of...start out and see how you do...you can re-eveluate later.

hope any of this makes sense.

Thanks Ron, thats very helpful.

I'm not too worried about having trained to the distance of the long bike ride as it is a huge charity event where people just 'potter' from A to B with large numbers of rest stops etc. I hoped to use it as a 'long bike' session and am happy to rest along the way if required. I do however want to 'run' the half marathon so am keen to make sure I've trained to that distance (2 hours is my optimistic goal!).

As far as overtraining is concerned, I do think that might be a risk but I know I wont follow any plan slavishly and will miss sessions so feel it is a relatively low risk. I have followed the 10% rule effectively since starting serious training at the end of last year and it has served me well, so will stick carefully with that.

I want to change the daily routine slightly and make more use of extra time on Sat & Sun and train 6 days. My plan was to move the plan back a day eg Long Run becomes Saturday and then move short swim to Monday to give something like:

Mon - short swim
Tues - long swim/med bike
Wed - short run
Thurs - long bike
Fri - off
Sat - long run
Sun - short bike

I'll put some numbers in and see what it looks like. I may drop the extra bike session depending on how it goes with workload and other commitments etc. My last sprint tri looks like being 26 Sept so am hoping for a 1' 15m finish compared with my 1' 32m last weekend.

Thanks