Thanks everyone for the suggestions, I've been trying different feeds and speeds and more flutes as suggested and I have improved things by running 4 flutes at minimum speed (about 300 RPM I think) - feed doesn't seem to make too much difference except that it has to be fairly slow at that speed to avoid machining marks. Locking the quill doesn't make too much difference but I assume the resultant cutting force is downwards which is against the feed mechanism so it's fairly rigid in that plane.

I wouldn't even consider climb milling with this rig :-)

Looking at the milling head, the part of the casting that connects the main body of the machine to the spindle is just two thin vertical plates - I might try and bolt some cross members in to stiffen it.

I think I can make some improvements with work holding as well though the vice is good and solid.

I'll also try and source some 6 flute cutters - any recommendations?

Thanks again,
Mark