I think your best bet is to start with the solicitor. Set out values as at date of purchase , work out what if any contributions( purchase price/stamp duty) each made, work out values of the site to be severed ( i.e. to go to you). Take preliminary advices. Then with that info, approach a good recommended accountant ( it may be that your earlier accountants were drip fed information on a "need to know/wait 'til they ask" basis)
I don't think it is complicated but I suspect you will find certain tax liabilities looming.
mf