Things may have changed since July 2004 when we came through Iran / Pakistan, but we got our visas (same day) at the Pakistani Consulate in Zahedan, which is just before the Iranian border. The officials were falling over themselves to give us a visa (a bit of cricket talk always goes down a treat), and I don't think that being NZ passport holders made any difference in this case... It did make a difference when applying for the Iranian visa's in Ankara!
I recall e-mailing some information about this to Chris Stone at the time he was trying to get visas. I'm pretty sure he posted it somewhere on the HUBB, so if you do a search you should find it... Probably in the ASIA forum.
We've been getting our visas en route, so haven't had any experience extending visas acquired in the UK.
To be honest, we just assumed that we could get Pak visa's in Zahedan since there was a consulate in the city. Perhaps a little naive of us, as it would have been a bummer if we'd had to ride all the way back to Tehran or worse still, the UK!
Indonesian visa's have been the most annoying to get so far... But the bikes were loaded onto a cargo ship in Penang today, and we follow them on a ferry tomorrow! Yay!
Hope this info helps, and hope you have as much fun on the road as we have been!
Jacqui and Trent