i was driving today off boost when check engine light came on, so i pulled over and poped the bonnet could smell a slight smell of burning rubber,  so i turned off the engine for 5 minutes then when i started it back up it was fine and light when off.........(could it be a slipping belt)   i would be most grateful if you guys could share your thoughts

I would have either said diff oil leak or the belt's that has given you the smell of rubber I doubt very much it will be the main belt's so either the steering or Aircon belts would be my guess 

The check engine light shouldn't come on with a slipping aux belt.

Check the belts anyway, but also check for codes stored on the ecu from the check engine light.

Oppps I did read the light came on. As Tlag pointed out, I'd get that checked with a reader if you can use one :) 

Cheers mate for pointing that out appreciated. :) 

i've had a few gearboxes blow on me over the years and the first one that started leaking gave the smell of burning rubber when the gear oil gets hot, as said it's odd for a belt to put a light up, any items with fluid will usually have sensors so that could bring the light up if theres a loss in pressure or fluid, again as said i'd try and get it scanned with a code reader if it's got the obd 2 port or under the dash with the black plugs using the flashing check engine light for code reading as per here

the car has run fine since very odd, may of been some bad fuel

did you check the codes stored?

I'd just see how it goes keep an eye on the levels of the fluids still. Just one of them random spikes or old codes like jay said :) 

no i dident bother like i say run sweet since done over 700mi with no probs (when i go over to see my old man hes going to plug it in)

Be worth checking but see it's being sold now so be worth clearing the codes if you get chance :) And it's a classic shape you can do it all without a reader 

http://uk.subaruownersclub.com/forums/topic/2366-manually-reading-ecu-codes-classic-impreza%E2%80%99s-only/