NEC Contract? Option B? It's remeasurable. I would push for a lump sum/ activity schedule (Option A).
Risks I see are:
Site clearance is an allowance. If more than one day, this will increase per day by the sum.
Grab load allowance - As above for the allowance, how much is a grab load? I would want a lump sum for this and the site clearance. How much vege' are they clearing?
Reduced level dig - Is the site already level? i.e. a scrape with a bucket. Or is the site signifcantly undulating? If it's flat then I would ditch this and go with a single JCB + driver for a 2 days at around £1500 allowance. Also, is the 200m3 correct? = 500m2 clearance area for 400mm.
Spoil removal is 80m3 - What they doing with the other 120m3? Is that the trim formation level? i.e. landscaping.
Drainage and water supply look OK for risk. Prices seem reasonable but I'm not Cambridge based so won't claim to know.
Off-site drainage - This can vary a lot and depends on your site plan. Does it include traffic management etc?
Off-site services - That doesn't look unreasonable to me as a sum. And you look fairly protected from risk, it'll only change if the meterage goes up. It's always worth getting them out to site first so they can measure up themselves and take decisions. That way, if they argue a change then you have the security that they had a site visit to include these things and price accordingly, which places it as ther cost.