I have just filled in a large pond.I Drained the pond and removed the liner. I used bricks and stones to line the bottom and the bought 4 tons of top soil and a ton of soil improver and rotted horse manure.After all this had work and cost the recent rain has created a muddy bog in the pond hole, what can I do to make this fit for planting.
An easier solution to hauling it all out may be to make a raised bed there. The bottom will likely remain a water reservoir.
From your descriptions I suspect that the location was always boggy, and a previous owner made a virtue out of a necessity and put in a pond.
A raised bed either formal (plank or concrete block) or informal (rocks, logs) will allow the ex-pond to be the drain for the raised bed. I would suggest at least a foot elevation.
Why not make virtue out of necessity and plant a bog/damp garden? You could do worse than having a look at Beth Chatto's book Beth Chatto's Damp Garden: Moisture-Loving Plants for Year-Round Interest - https://www.amazon.co.uk/Beth-Chattos-Damp-Garden-Moisture-Loving/dp/1844030458. Here's a couple of RHS links for bog gardens: https://www.rhs.org.uk/advice/profile?pid=356 and https://www.rhs.org.uk/advice/profile?pid=807