There is a spares catalogue here which lists your toolholder and gives the order number for a replacement screw. The UK agent is
Seco Tools (UK) Ltd
Adams Way, Springfield Business Park
Alcester
Warwickshire B49 6PU
Tel: 01789 764341
Fax: 01789 761170
You might be able to bring the head off the screw with a Dremel, or maybe unscrew it with a left hand drill bit, or even hammer a bit of copper or brass rod into the screw head and use that to unscrew it.