A quick update since I posted this 2 weeks ago - have been following all the advice, kept them separated then introduced the pups one at a time (holding them in my arms) and letting the old dog sniff them. Did this for a few days just 20 minutes at a time. Then I let the pup down and after a few initial barks from the old dog, they actually started to play! I am now able to bring the 2 puppies in to the kitchen with the old dog and they play quite happily for about 30 minutes, at which point the old dog goes to the door to escape.
Yesterday I put all three out in the garden together and they chewed bones and played together for about 2 hours - happy days indeed! Going to do the same again today.
The only problem remaining is when the old dog tries to go the the toilet, the pups annoy her, so I keep them in their crate when old dog needs to go out.
In a week after their final vaccinations, I will be able to let them walk around the fields together and I think this will be a big help also. But, thanks again for all the advice, it is certainly working, slowly but surely.