My understanding and I have been going through very protacted Ancillary Relief proceedings followed by many claims by my ex husband (I divorced him just over a year after he left me and moved in with his girlfriend - btw he did contest that he was having an affair but subsequently married the lady last year) is that even though you are not actually divorced your spouse cannot attempt to claim as a marital asset assets that may come to you as a gift or inheritance or attempt to divorce you once you receive the inheritance stating that they are marital assets if you receive them up to three years before the marital breakdown.
Also your wife having accepted that the marriage is over (you do not state whether you or she had the affair but as you are married I do not believe it is relevant) and you have been separated for 6 years she can hardly say when you had no assets at the point that you split and therefore is presumably supporting herself that she is now due a chunk.
You also say that your mother is the sole beneficiary of your fathers estate on his death - so I assume that there is a will.
Is the maintenence you are behind on maintenence for your wife or maintenence for the children?
It makes sense to divorce and settle the finances sooner rather than later. Every case is different and inheritances aren''t black and white.
After 6 years you can divorce on the basis of 5 years separation without the consent of your wife. There is no reason why you can''t do this yourself all the forms you need are available to download from the Ministry of Justice website or you can use an online service if you don''t want to spend time doing some research and filling in forms.
The finances are more complicated but if your situation is straight forward you can do it yourself too or just consult a solicitor for the financial side.