I have managed to resolve the 5 baud issue & progress my diagnostics project a little - thanks for the pointers guys. I am now able to initialise comms with my engine management ECU using KWP2000, start a diagnostics session & negotiate security by requesting the seed and returning the key. Although my site is geared towards the Range Rover, the info may be of use to other people attempting non-obdii comms with their own ECUs.
I have posted some pictures here:
The interface between the PIC and the ECU over the K and L lines is trivial, all the details needed are in the ELM327 datasheet, Although the ELM chip itself couldn't be used for what I was trying to achieve.
Alt4 - The Open Source Range Rover Diagnostics Project