problems with table mode

[Pete Phillips <pete@smtl.co.uk>]

I am having a problem with a table I use to calculate my annual leave


| Basic Entitlement |         33 |
| Carryover         |          5 |
|-------------------+------------|
| Date              | No of days |
|-------------------+------------|
| 25th April        |          1 |
| 26th - 28th May   |          3 |
|-------------------+------------|
| Used              |          4 |
| Remaining         |          4 |
#+TBLFM: @III+1$2=vsum(@II+1$2..@III-1$2)::@III+2$2=@1$2+@2$2-@III+1

The table gives me the correct calculation for 'Used' - i.e., the number
of days I have taken so far.

However, the formula for the 'Remaining' row gives me the error 

  org-table-get-stored-formulas: Double definition `$2=' in TBLFM line, please fix by hand 

I don't understand this - if @III+1$w works for the second column in the
Used row, why doesn't @III+2$2 work for the second column in the
Remaining row ?

Any help appreciated.

Regards
Pete