If it doesn't go over 70 ever than just replace thermostat. Though 10 minutes are not really terribly long for it to warm up (as long as gauge goes to 90).
I agree, replace the thermostat and go from there.
Idling a cold engine causes more wear than driving with a little care immediately after starting (and it really pisses your neighbours off - I want to smack my neighbour for idling his crappy 90s diesel in the courtyard every morning at 7).
I think the committee is out on that tbh. I thought the ECU compensated for starting a cold engine and more fuel is injected etc. So more juice is used whilst driving a cold engine, although to be fair this is probably infinitesimal amounts. metal expansion and contraction is also something to consider.
I was under the impression its quite important to do it with diesels, turbo'd in particular?
I don't mind sitting in a cold car for a few minutes in the morning, that's what heated seats are for. 
I wish my seats were up to the job! takes forever and a day to get my seats warm, and theres nothing worse than cold leather in the morning!