Free space at top of partition table

Hi, I'm running OS X Mountain Lion on a Late 2009 iMac. Over the last few days, I have noticed that something has gone horribily wrong with my internal hard disk. The free space on my drive, instead of being at the bottom of the table, is at the top.

I've got no idea how I managed to do this, but I was wondering if anyone knew how to fix it. Running

$ sudo gpt -r -vv show disk0

outputs:

gpt show: disk0: mediasize=1000204886016; sectorsize=512; blocks=1953525168

gpt show: disk0: PMBR at sector 0

gpt show: disk0: Pri GPT at sector 1

gpt show: disk0: Sec GPT at sector 1953525167

start size index contents

0 1 PMBR

1 1 Pri GPT header

2 32 Pri GPT table

34 6

40 409600 1 GPT part - C12A7328-F81F-11D2-BA4B-00A0C93EC93B

409640 1241064104

1241473744 710781848 2 GPT part - 48465300-0000-11AA-AA11-00306543ECAC

1952255592 1269536 3 GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC

1953525128 7

1953525135 32 Sec GPT table

1953525167 1 Sec GPT header

and

$ sudo diskutil list

gives

/dev/disk0

#: TYPE NAME SIZE IDENTIFIER

0: GUID_partition_scheme *1.0 TB disk0

1: EFI 209.7 MB disk0s1

2: Apple_HFS Lion Client 363.9 GB disk0s3

3: Apple_Boot Recovery HD 650.0 MB disk0s4

Any help would be appreciated. Thanks 🙂