Thanks for your help lemming. The issue is now fixed
The 3.3v out on the mega seems happy supplying the constant 75ma that my multimeter says the module is drawing.
The resolution was in the end connecting the VCC to the 3.3v, I had mis-understood the description and had been working on the assumption that the power input was 5v tolerant as well as the data pins.
So to finalise this in a nutshell:
Fix the ENC28J60.c file as follows:
#define ENC28J60_CONTROL_CS 53
#define SPI_MOSI 51
#define SPI_MISO 50
#define SPI_SCK 52
Connect The pins as follows:
Atmega pin ENC28J60 module pin
50 4
51 5
52 6
53 7
3.3v 9
Grnd 10