Many thanks, everyone. It's reassuring to hear what you say!
The field I took off from is 3300feet and with an OAT of 31 degrees that gives quite a reasonable density altitude. Sounds like I should be leaning on take-off with a full power run-up.
Also, in these conditions it isn't really necessary to use carb heat at all and frankly the engine does not like it.
For info, my friend's 90HP PA18 has its C-90 wired full rich and it didn't like it by the time it got to 10,000 feet, coughing a bit etc.
Ah, the adventures of lowlanders in the mountains!
Thanks again,
David