Data validation - Cell to increment by 1

hi guys,
I know this is probably very simple, but I have been trying for hours to no avail..

Basically, I have a column header named invoice number at the top of column B (B1)

This column then contains incrementing numbers, with some blank cells inbetween

i.e.

B2 :- 467
B3 :- 468
B4 :- empty
B5:- 469
B6 :- empty
B7 :-
.
.
.
.
.



Basically, I would type 470 in cell B7 in order to continue the series, however, I would like some validation to take place on the cells in this coliumn to only allow a number which is 1 more than the last non empty field, ..i.e in this example it would only allow the user to enter 470 in B7 (by referencing the contents of the last non empty field; B5 (469) and adding 1 to it... (if any other number other than 470 (in this example) was entered, it would report an error)

Im sure this can be done with some conditional forumula? any help would be greatly appreciated...

many thanks mate works a treat

what is that forumla translated into english?