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Henrik Thank you very much for your assistance. It looks like I need to spend more time and effort looking at and analysing my configuration registers. Though as you suggested it appears that if I set the various interrupt registers (INTCON and PIE1) you do not need the @ INT_ENABLE TMR1_INT and @ INT_ENABLE AD_INT lines (commented out below), maybe Darrel could shed some light on this? Though I must thank him for the DT-Ints code as it does appear to be excellent piece of code.

To confirm I initially intended to have the analogue attached to AN0 but moved it to AN1 due to realising that I could not have CMCON0 (bits 2-0) set that AN0 as an analogue input and GPIO.1 as a digital input and output. so I swapped them around and unfortunately didn't swap all of the settings in the configuration registers that I should have done.

I have attached the working code below for anyone to reference in the future.

Code:
clear
DEFINE OSC 8

'GP0 LED OUT
'GP1 AD1 FOR POT INPUT
'GP2 LED STATUS
'GP5 LED STATUS 1

'REGISTERS
OPTION_REG = %11000000
INTCON = %11000000
PIE1 = %01000001
PIR1 = %00000000
OSCCON = %01110001
TRISIO = %00000010
ANSEL = %00100010
CCP1CON = %00000000
CMCON0 = %00000100
ADCON0 = %00000101

INCLUDE "DT_INTS-14.bas"     ' Base Interrupt System
INCLUDE "ReEnterPBP.bas"     ' Include if using PBP interrupts

;---------------------------------------------------------------------------
wsave   VAR BYTE    $20     SYSTEM      ' location for W if in bank0
;wsave   VAR BYTE    $70     SYSTEM      ' alternate save location for W 
                                         ' if using $70, comment wsave1-3

' --- IF any of these three lines cause an error ?? ------------------------
'       Comment them out to fix the problem ----
' -- Which variables are needed, depends on the Chip you are using -- 
wsave1  VAR BYTE    $A0     SYSTEM      ' location for W if in bank1
'wsave2  VAR BYTE    $120    SYSTEM      ' location for W if in bank2
'wsave3  VAR BYTE    $1A0    SYSTEM      ' location for W if in bank3
' --------------------------------------------------------------------------

'OUTPUT PINS
LEDOut VAR GPIO.0
LEDStatus VAR GPIO.2
LEDStatus1 var GPIO.5

high ledstatus
pause 500
low ledstatus
pause 1000
high ledstatus
pause 500
low ledstatus
T1CON = %00110001

'VARIABLES
LENGTH VAR WORD
PauseA var byte
PASS var byte

ASM
INT_LIST  macro    ; IntSource,        Label,  Type, ResetFlag?
        INT_Handler    TMR1_INT,  _StateChange,   PBP,  yes
        INT_Handler    AD_INT,  _PulseLength,   PBP,  yes
    endm
    INT_CREATE               ; Creates the interrupt processor
ENDASM

'@ INT_ENABLE TMR1_INT
'@ INT_ENABLE AD_INT

ADCON0 = %00000111

'MAIN PROGRAM
MAIN:
pausea = 0
WHILE PauseA <= 100
    PAUSEA = PAUSEA + 1
    pause 1
wend
toggle ledstatus
GOTO MAIN

'SUB PROGRAMS
PulseLength:
LENGTH.highbyte = ADRESH   'SET UP THE TIMER SET UP VARIABLES
LENGTH.lowbyte = ADRESL
toggle ledstatus1
@ INT_RETURN ;RETURN

StateChange:
if pass = 5 then
    TOGGLE lEDOUT
    pass = 0
    else
    pass = pass + 1
endif

TMR1H = LENGTH.highbyte 'Transfer the timer variables to the timer register
TMR1L = LENGTH.lowbyte
T1CON = %00010101
ADCON0 = %00000111
@ INT_RETURN ;RETURN

END
Thank you again for your assistance, you have been a real help.

Andrew