And (at least yesterday, didn't check today) all fares quoted on the web site 'special offers' were higher than in the email (or slightly lower but excluding tax). What is going on here?

[ADDED LATER] Now there is a link on the KLM home page 'North America Fly from 15 UK airports from only £257'. Once again in complete breach of advertising standards as there isn't a single flight in there for £257 or less (and no small print about excluding tax, but that does appear when going to the actual offers page). Also the email stated from £287 including taxes, as these are e.g. £134 to SFO that doesn't make any sense at all*. Interestingly enough New York is missing in the list, I wouldn't be surprised it will turn up tomorrow or next week and will be priced at £257... Then again most prices quoted here are higher than in the email, so something doesn't look right here. Seattle is a positive exception (unless the email fares are inclusive of tax as they seem to be intended).

The fare conditions (except a Sat/Sun night stay away) are not to be found anywhere, validity period isn't stated (but probably September & October).

Fare conditions from the link in the booking page:
Changes are not allowed
Cancellations are not allowed
Conditions Apply

Extremely useful

* or do they really mean taxes, not taxes & fees, additional airline income, fuel surcharges, etc. etc. That would be a first for any airline