I finally got into the office today and had a chance to put the transmitter on the bench. First job was to swap out the filter components - though actually the new filter isn't that great either (the roll off needs to be steeper) so I might change it again one day...

But the next thing was to measure the output power: a measly -2dBm! Barely 0.5mW! And that's after I changed Rset for a 2kohm to increase the output power. No wonder I'm struggling to get the signal out...