Thanks to all who replied - your estimates were pretty close. We
arrived Paddington at 0830, had no wait at all for a taxi, it took
about 20 minutes and the fare was £12.50 including tip. Much, much
less hassle than the tube.

I did take the tube for the return trip, and can recommend it to/from
St. Pancras - close to Eurostar exit, well signed, lifts, and no real
problems.