If your partner has bad credit and you have good and you enter into a financial agreement together - you don't automatically inherit her bad credit name I don't believe. However, you are linked by the agreement so should her bad credit continue - you'll be linked in that way as it'll be under both of your names.
If for example, your partner is �10K in debt and you take out a mortgage, I believe she could add another �10k on to the mortgage to pay off the debt and you can have something written into the mortgage by the solicitors that the additional �10k belongs to her - and it can be worked out that she has to pay slightly extra than you per month for the repayments.
That's only my guestimate though - if you're never sure with these things it's always best to speak to somebody like an independent financial adviser, as they'll be more aware of how it works.