Practically, do you know which bit of land is which?
I would that that the original plot was freehold and additional land was bought in which happens to be leasehold or vice versa unless there is an element of flying freehold which has been incorporated (a freehold which hangs over eg in jigsaw housing).
Have you asked about the possibility of buying the freehold of the leasehold part to see if that is feasible?
You should be advised as to any obligations, restrictions etc... on all parts of the land, be it freehold or leasehold and you should be aware of which obligations, restrictions etc... relate to which bit.
I'd say the first step would be to find out which bit is which to put it in a more practical setting. Has your solicitor send you copies of the title plans of the freehold, leasehold and the superior title to the leasehold?