When placing a bid, you always have to follow the increments of the bidding ladder. If the leading bid is 30 GBP, for example, then you need to bid at least 35 GBP. You can't bid 34 GBP.
However, when placing a high max bid at an early stage of the auction, a bidder may enter any odd amount. The automatic bidding agent may later place automatic bids on their behalf that might not correspond to a full step increase in the bidding ladder.
Example:
Bidder A is prepared to bid 36 GBP for an item that hasn't received any other bids yet. She enters a max bid of 36 GBP. A bid of 30 GBP (the start amount) is then placed on her behalf.
Bidder B sees this bid of 30 GBP. He places a 35 GBP max bid, resulting in a 35 GBP bid on his behalf.
The next bid step is 40 GBP, but our automatic bidding agent is not allowed to bid more than 36 GBP on behalf of bidder A. Therefore, a bid of 36 GBP is placed on behalf of A, and she now has the leading bid again.
We always ensure that the person with the highest bid amount gets the leading bid, even if it means that we sometimes can't follow the bid ladder.