A Mesher order is kind of Court order, named after the case where it was first used.
What it means is that there is an order for sale of a property, usually the marital home, whereby the sale is postponed until the occurrence of one of a number of specified events, usually, but not invariably, where children reach their majority. Until that happens, one of the spouses, almost invariably the wife/mother, has the right to live there, to the exclusion of the other. At this point the house is sold and the purchase price is divided between the former spouses in the percentages specified in the order.
A legal charge is - well, I suppose you could say it implies a situation where one party is the owner of property, but some third party ( usually a mortgage lender ) has contributed in some way to the purchase price. As with a Mesher order, the money becomes payable in the occurrence of specified events, usually a sale, or perhaps a default. At that stage the money becomes immediately repayable, and the person entitled to the benefit of the charge can enforce a sale.
You asked :
a) end up with reduced mortgage capacity if I wish to purchase a new property?
Your ability to get a new mortgage could well be prejudiced, perhaps because you could remain liable on the original mortgage of the family home, perhaps because there are two households whereas previously there was only one.
b) have to pay anything towards the mortgage or upkeep of the FMH?
You should not have to assume direct responsibility for the mortgage on the family home, but you could be paying indirectly through child/spousal support. The primary responsibility would always be with the occupier, who would be expected to carry out all decoration and routine cyclical maintenance. There can, however, be issues concerning major repairs, insurance, and responsibility for the capital content of the mortgage repayments. From what I can see, there is no norm here and it''s a matter of individual negotiation, unless the Court makes an order.
c) have responsibility in case my ex is not able to maintain mortgage payments, i.e. will my credit score be affected?
The answer is that you could do. If you were a party to the original mortgage, the odds are that you would remain liable, though this is not invariably the case.