Guys thanks for your continued input.

using the #hum/100 gives 87% as the value, which to me seems rather humid, especially when the indoor humidity according to my maplins weather station says it's 51%. As a comparison I tried the following code which uses the pause and pulsein statements to read the chip

Code:
ASM 
  __CONFIG    _CONFIG1H, _OSC_HS_1H
  __CONFIG    _CONFIG2L, _PWRT_ON_2L  
  __CONFIG    _CONFIG2H, _WDT_ON_2H & _WDTPS_512_2H
  __CONFIG    _CONFIG3H, _MCLRE_ON_3H & _LPT1OSC_OFF_3H & _PBADEN_OFF_3H  
  __CONFIG    _CONFIG4L, _LVP_OFF_4L & _XINST_OFF_4L
ENDASM


'*******************************************************************************
' LCD (20 x 4) set up
'*******************************************************************************

DEFINE LCD_DREG  PORTB                  ' LCD Data port
DEFINE LCD_DBIT  0                      ' starting Data bit (0 or 4)
DEFINE LCD_EREG  PORTB                  ' LCD Enable port
DEFINE LCD_EBIT  5                      '     Enable bit  (on EasyPIC 5 LCD)
DEFINE LCD_RSREG PORTB                  ' LCD Register Select port
DEFINE LCD_RSBIT 4                      '     Register Select bit   (on EasyPIC 5 LCD)
DEFINE LCD_BITS  4                      ' LCD bus size (4 or 8 bits)
DEFINE LCD_LINES 4                      ' number of lines on LCD
DEFINE LCD_COMMANDUS 2000               ' Command delay time in us 
DEFINE LCD_DATAUS 50                    ' Data delay time in us 

'*******************************************************************************
' Defines Statements
'*******************************************************************************

DEFINE  OSC 20                          ' 18F4520 / 18F2520, 20mhz crystal
ADCON1 = $0F
clear    

'*******************************************************************************
'Analog and Comparator settings
'*******************************************************************************

ADCON0 = %00000000                      'AD converter module disabled
ADCON1 = %00001111                      'All Digital
ADCON2 = %00000000
CMCON = 7                               'Disable Comparators

'*******************************************************************************
'Port and Register settings  (interrupts)
'*******************************************************************************
    
TRISA  = %00010111              
TRISB  = %00000011               
T0CON  = %11000111

T1CON = %00000001                               ; free-running, 1:1 prescaler
TMR1H = %11111111
TMR1L = %11111011


'*******************************************************************************

LCDOUT $FE,1:FLAGS=0:PAUSE 250:LCDOUT $FE,1:PAUSE 250 ' Initialize LCD

'*******************************************************************************

dht var byte[32]
humidite var byte
haut var byte
bas var byte
temp var byte
x var byte
dht11 var portA.1

'*******************************************************************************
start:
TRISA.1 = 0 '
high dht11
pause 2000 ' wait 2 sec
low dht11 : pause 18' send 20ms low
high dht11 : pauseus 30 ' send 40us hi
TRISA.1 = 0
PulsIn PORTA.1, 1, haut
if haut < 15 then goto start
for x = 31 to 0 step-1
PulsIn PORTA.1, 1, dht[x] ' 1
next x
For x = 31 to 0 step-1
if dht(x)>=9 and dht(x)<=21 then dht(x)=0 'if pulsewidth between 20 and 40uS then read as '0'
if dht(x)>=29 and dht(x)<=41 then dht(x)=1 'if pulsewidth between 60 and 80uS then read as '1'
next x
humidite=dht[31]*128+dht[30]*64+dht[29]*32+dht[28]*16+dht[27]*8+dht[26]*4+dht[25]*2+dht[24]*1
temp=dht[15]*128+ dht[14]*64+dht[13]*32+dht[12]*16+dht[11]*8+dht[10]*4+dht[9]*2+dht[8]*1
lcdout  $FE,1 
lcdout  $FE,$C0,"Humidite = ",#humidite,"% "
lcdout  $FE,$80,"Temperature = ",#temp,$DF,"C"
goto start
This gives 174c in our living room, with the humidity fluctuating between three values of 24%, 144% and 152%. Clearly there is something wrong with the hardware. I'm using a new solderless breadboard so I'll look at hard wiring the sensor and see if that helps, even though the connections between the Easypic board and sensor are next to each other