Not a problem!
Sorry for extra questions but can we use it to get from the airport - Narita to Tokyo on 1st April?
Yes - the Japan Rail Pass is valid on all JR services into Tokyo, including the Narita Express direct train into town.
We had then hoped to spend some time in and around Tokyo and perhaps make a day trip to Kamakura and another to Mount Fuji. Would we be likely then to make enough trips including the one down to Kyoto on the 7th April to make buying the 7 day pass worthwhile?
As Kamakura return is only around ¥3000, how about a trip to Nikko? The return will be around ¥10,000 (incl bullet train). Nikko it's a great day trip from Tokyo and will get you much closer to your ¥28,300 target. Here's a sample itinerary. If you went to Nikko, Odawara or Kawaguchiko for Mt Fuji/Hakone in addition to your Kyoto leg, you'll make your ¥28,300 target and also give yourselves lots of flexibility to change plans.
Also can we use it on all the trains in and around Tokyo? It would be very useful to be able to get on and off local trains without having to worry about buying tickets.
The Japan Rail Pass is valid for all local JR trains around Tokyo. Here's a map of the JR lines in Tokyo for reference. There are other rail companies also operating in Tokyo, so you'll have to be a little bit aware of the service you're going for, but the JR sign is most prominent so you should have no problems identifying the stations. You may also find buying a Suica or Pasmo subway card is a handy additional ticket to purchase once you are in Japan. The Subway is inexpensive and is a good addition to the JR lines.
Hope this helps!