How badly will my credit score affect our mortgage?

My partner and i are going in for a house next year. He earns around £100k a year, and I earn around £30k.
His credit rating is basically perfect - over 900 and he has an existing mortgage for around 30k.
Mine is non existent at 300 (i had a lot of defaults in the past due to health problems and was signed off work)
i'm really worried I'm going to stop us from being able to get a house. :( how badly will this affect us? I'm trying everything to build up my score - and i haven't missed any payment in two years but its still looking crap.

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