Thanks for the post SurferTim - the problem was due to the IDE version. I upgraded to 1.0.1 and hey presto...it works. Out of curiosity I thought I'd see what had changed and found that the initial DHCP_DISCOVER message has been modified as below.
01 01 06 00 00 00 03 31 00 00 80 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 AA BB CC
DE 02 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 63 82 53 63
35 01 01 3D 07 01 00 AA BB CC DE 02 0C 0C 57 49
5A 6E 65 74 43 43 44 45 30 32 37 06 01 03 06 0F
3A 3B FF
The main difference I can see is in the creation of the hostname which was intended to be "WIZnet" concatenated to the last 3 bytes of the MAC address. In the earlier version, the binary MAC address values were used, resulting (I guess) in a non-printable hostname. In the later version, the binary values have been converted into their hexadecimal equivalents, and then concatenated as the character representations, i.e. 0xCCDE02 becomes the character sequence 'C', 'C', 'D', 'E', '0', '2', resulting (in my case) in the hostname "WIZnetCCDE02".
Thanks again for helping me with this problem.