I've just put some marginals in the pond which came from lincolnshire pond plants and am waiting for them to get going. What I'm after is suitable plants to go in pockets between rocks by the watercourse. It's a new fibreglass rock effect one which needs to weather in yet but has rocks around it now.
I used a company called Puddle Plants, when I planted up the pond, last yea. They were so knowledgeable and helpful over the phone.
Their website is great and will give you all th info you need regarding marginal plants etc.