OK, back with GH book.
The following quantities are for 12 people.
They then say to multiply these to fit the number you're expecting as follows:
25 people multiply by 2
50 multiply by 4
75 multiply by 5.5
100 multiply by 7
Salads
Tomato allow 700g (1.5lb)
Salad leaves, allow 2 medium heads
Cucumber, 1 large
Dressings
French Dressing allow 175ml
Mayonaise allow 300ml
Bread
Fresh bread, allow 1 large loaf
Medium sliced bread, allow 1 large loaf
Cheeses
For cheese & wine party, allow 1.5kg
At the party I did (60 people) we got some large, cold meat pies from our butcher, eg turkey,pork & cranbury (5 which each cut into 12 pieces). I made various pasta/rice/cous-cous style salads, I think there were about 6 bowls (2 of each type), each of which were 12 servings.
I did cheese and biscuits rather than sandwiches. Buy big lumps of a few cheeses rather than lots of bits, leftover cheese freezes well and crackers keep for ages. We bought some french sticks too (5).
I also made a load of canapes from Delia's Christmas book, almost all of them can be done ahead. According to Delia, if you're serving 'canape' style stuff at a drinks party you should allow 8 per person. All the leftovers were eaten by the time the tidying up had been done.
Hope that's some help, my Good Housekeeping book is on the pile to go to the charity shop as I've been decluttering so I'll happily send it to you if you want !