I'm not familiar with the BS5930 specification, but in other classification systems I think this would be called a clayey SAND and GRAVEL, instead of the a silty one.

I don't use BS5930 - it is almost as convoluted as ASTM D2487. How I long for the Burmeister system as per Canada's NRC (National Research Council) and used by the older geotechnical firms in Canada . . . anyway, I digressed. In viewing the constituents you have, I would classify it as a "very silty SAND and GRAVEL" given that the fines content is 20% and would use the "silty" as modifier in that for a PI of 12 you are likely in the ML or MI range on the Plasticity Chart. If the plasticity were to be in the CL or CI range, I would replace "very silty" by "very clayey". Typically, my experience with Canadian tills is that the fines are silty.

What I would have used, from my practice, is "Sand and Gravel, some Silt, trace clay - TILL".

Hope this helps . . . and by the way, BS5930 uses a totally different set of Undrained shear strength values in kPa (kN/m2) for clayey soils compared to normal North American practice and as indicated in Terzaghi & Peck, and many others. Leads to difficulties from one country to another - for many overseas projects, one would need to know where the geotechnical engineer was from in order to determine what the terms mean.

BS Normal
Very soft 0 - 20 0 - 12.5
Soft 20 - 40 12.5 - 25
Firm 40 - 75 25 - 50
Stiff 75 - 150 50 - 100
Very Stiff 150 - 300 100 - 200
Hard > 300 >200

Don't forget that the Atterbergs are only carried out on the fraction that is less that 425 microns. So the PI you mention doesnt describe the behaviour of the material as a whole. You've probably had 60% of it seived out.
Aren't we meant to be using the Eurocodes now anyway? ;oD

@Stego1, don't think your point is correct about the effect of the Atterberg limits. Many highway specs put a limit on the PI of, say, a base course - typically 6 or less. If the minus 425 fraction has a PI of 10, then it does not pass the spec - you don't go and say, well, the minus 425 is only 20% of the whole so the "effective PI" is 1/5 of 10 or 2 so it passes. Specs and descriptions developed know full well that the Atterbergs are done on the minus 425. What they are saying, while the material is only a small part of the whole, the fines do play a major role in how the whole behaves. That's my take on it.

Sorry I probably didn't make my point clear. I meant that when describing the soil, you don't call it a clay on account of the <425 fraction having plasticity. You call it a clay if the material as a whole behaves as a clay when you poke a finger in it or throw a lump at an excavator bucket. If it behaves more like S&G then you describe it as such. You don't rely entirely on grading or atterbergs to classify the material.

As for dividing the PI by 5 because the <425 only makes up 20% of the sample.....that sounds like "structural engineer's logic"!! (Tongue in cheek, before any SE's kick off).

I take your point Stego1 - no one would (or should) never call this a clay - however, the way things are set up, if the fines are plastic enough, they love to add "clayey" to it. We would have just called it "trace clay" or "some clay" depending.