Hi Jennie - registry office twin!!
I'm getting married at Morden Park House too in September 2012! Its a beautiful place.
We're going to the Antoinette Hotel in Wimbledon for our reception - its only 10 mins away in a taxi. the outside area isnt great - but we're planning to have our photos at the registry office in the grounds, and then also have some in the church gardens opposite the hotel - its smaller but beautiful.
the hotel also does good deals for different packages - I would def reccomend looking at them to see what might suit you!
and also - we're having a buffet too - at £17 per person :-)
good luck and congrats xx