Thanks Ramona and Jolene for the tips on bobbin tension and needle burrs (never thought of burrs). Maybe I should summon up courage to have another go.
My attached photo shows the needles I accumulated for the Pfaff. You can see there are sizes 90, 100, 110, 140 and 200.
Also a photo of my thread - V92 poly.
Any opinions on which of the needles in the photo would best suit the thread, or does needle choice depend more on the fabric (Sunbrella type) than the thread?