Puree the strawberries with the lemon juice (you can use a liquidiser or food processor to do this or you could mash them with a fork, but be careful not to leave them too rough as they can get a bit icy). Push the puree through a sieve to remove the seeds.
Place the sugar and 200ml water in a saucepan, then heat slowly, stirring to dissolve the sugar. Boil fiercely for 5 mins, until it thickens and the last drops off a spoon dipped into it form a kind of thread (i.e. the drip off the spoon really slowly). If using a sugar thermometer, this will read 107C/225-226F.
Meanwhile, using an electric whisk, beat the egg whites with the cream of tartar until stiff. Still whisking, gradually pour in the hot syrup in a thin stream, and continue to whisk until the mixture is cool, glossy and stiff (about 4 or 5 minutes). Fold in the strawberry puree and whipped cream, though not completely - I quite like to leave this slightly marbled and you get a better marble effect if you pour it around the outside edge.
Freeze overnight. This ice cream can be scooped straight from the freezer.