There are five sets of six characters. That would be thirty in all although one chap appears in several places in slightly different clothes or pose!
1501 is the set of pit mechanics and drivers.
The three missing characters are shown below.
1502 is the set of spectators
I have all of these.
1503 is the set of race track officials.
These are the most difficult to find intact. Most carry a post which gets broken or the paper flags get lost. There is also a broadcaster chap whose aerial usually gets broken. There are two similar-looking chaps holding flags in black leather coats and blue trousers but one has a yellow tie and the other a red tie. Update 30 September 2019 - I now have a complete boxed set with original flags still attached too! So those illustrated above are available now.
1504 is the set of press officials
There are three missing
You may see that this last odd-looking fellow is the one who appears in the garage attendants set below. Or maybe he has a twin.
Update 30 September 2019 - I have now learned from someone with sharper eyes than mine that the ugly chap in the brown leather coat is not actually the same as the ugly chap in the brown leather coat in the garage attendants set after all. Here is the correct ugly chap in a brown leather coat.
You'll see he has both arms free from his body and extended a little. So I do need him and he's not an identical twin.
Here is 1505, the set of garage attendant figures
With the 'twin' who I now see is not an identical twin, my set is complete.
I am not entirely sure which sets came with which Gift Sets. I had originally thought that the Silverstone Set came with all five but those I have seen for sale only have 1501, 1502 and 1503. I can't imagine that 1504 wouldn't have been included, though, as there is a Press Box kit in the big set. So I think it is only 1505 that was not included and, I think, you'll find that in the big Garage Gift Set.
The two chaps in blue overalls in 1505 also came in the box with the 1142 Holmes Wrecker model and the one who looks like he's changing a wheel also appears with some farm conveyor models. Update 30 September 2019 - again I have been advised that the Holmes Wrecker guys are slightly different!
If anyone reading this happens to have duplicates of any of the characters that I'm missing then do let me know. Otherwise I hope these illustrations help collectors figure out which they may need and who fits in where.