Quorn ordinary and cumberland sausages are syn free, as are the mince, meatballs and pieces. Burgers are around half a syn. Products in crumb coatings or ready meals tend to be higher in syns. If you do a search there is a list of syn values on this forum.
Asda meatfree products are very good (better than quorn in my opinion) - sausages are 1 syn each, mince and pieces are free, meatballs are half a syn for 1/4 bag.
Tesco meatfree sausages are the best but are 1.5 syns each. They used to do syn free fillets which were fab but these changed recently and are now about 3 syns each as they are crumb covered.