David,
Try Jenny Blackwell of J B Cutting Tools [phone 01246 418110] The screws are known as tip screws and can be supplied with the correctly fitting Torx head wrench, small ones with a 'flag' type side grip to limit the torque you can apply
3.5 mm x 0.8 mm is a standard metric thread and you should be able to run a tap down the female part to clear out debris
Brian