Injecting false positions could be much more fun. I know it has been done in the lab, but I'm not sure it has been demonstrated in the field.
We've got a couple of GPS signal generators at work which give a spoofed fix at a range of literally 3-5cm. Which is frustrating in the extreme. A 100 foot range would be brilliant and the fact we don't have it makes me think it's very hard to acheive or illegal. It can't be a problem of signal level because as we know the signal levels are tiny - below the noise.
Personally, I think spoofing a credible moving position on a vessel would be impossible unless you had feedback from the vessel. Spoofing a time and date on a fixed station in a bank might be easy if you had the range but surely these numbers are sanity checked with a local quartz clock and/or an clocks in other locations?
I find the idea of ships getting lost in the Thames Estuary or hitting each other in the channel because of lost or innacurate GPS laughable. They have radar and eyes. I wonder if David Last is touting for business of some kind with a shock story?