Looks like it is worthwhile.
Just got back from the gym and sat and read my own diary.
I've never kept a diary before and whilst the purists might argue this isn't much of a diary, it's quite interesting to see a progression and to see what I was thinking in the past.
The bit about exercise popped out - so I thought it worth recording an update, especially in the light of this evening session at the gym.
Basically, I've broken the 118 barrier - I break it regularly and quite easily.
Where I was going wrong before was in trying to "force" my heart rate up by a short - unsustainable - burst of activity. What I've learned is that my heart rate increases steadily till it's at the "right" level for the level of activity I'm doing. So a short burst - whilst it knocks the stuffing out of me -is too short a time for the HR (or maybe the HRM) to respond.
The way I like to structure my exercise now is :
Start on the treadmill - walking at 2.3% incline 4.5mph for 4 mins.
I then "jog" at 5.5mph. Initially I was jogging for 1 min - but I'm slowly increasing the jogging time and so reducing the walking time between jogs.
I then drop back to walking at 4.3 - 4.5 mph for 4 mins.
Then I'll jog for another minute at 5.5.
I repeat this, till I've been on the treadmill for 25 minutes.
(If I need to, I do a minute walking at 3.8 mph straight after the jog)
Finishing with the 5 min "cool down" period built into the machine.
I'm cautious about jogging too much at my weight, don't want to damage the knees - I've ordered a proper pair of running shoes, when they arrive I will try to push out the time I spend jogging as opposed to walking, as I see the difference it makes to my HR and Kcals burnt to go that little bit faster. But I'm in this for the long haul, so not I'm not desperate to run before I can walk

(Who knows, one day I might try and prepare for a 5k fun run - that would shock a few folks)
So, using the "technique" above - my HR now gets much higher than 118 -
I've seen it hit 131 - without me feeling like I was about to throw up or die or both.
I take a different approach on the cross trainer. On this kit - my HR climbs more slowly over the whole duration I'm on the thing.
Chatting with the trainers at the gym, I've learned that trying to increase my speed is actually more effective than trying to increase the resistance of the machine.
So finally - I'll record this evenings numbers here - so I can look back on them in a few months and see some progress (hopefully)
Treadmill - Duration 28 mins. Avg HR 114 Max HR 128 Kcal 240. 8 mins above 120.
Eliptical - Duration 36 mins. Avg HR 120 Max HR 130 Kcal 345. 27 mins above 120.
Whilst I am still a long way short of even the loosest interpretation of being "fit" - I know that I am fitter than I've ever been in my life. Just a shame I had to get to 48 to start doing this - Well done those of you on here in your 20's and 30's. You won't regret it.
So, to answer my own question - Is it worthwhile ? - clearly the evidence would seem to point to the fact that it is.