Hey there,
I was in this situation too when applying for mortgages about 6 mnths ago.
My employer could offer a letter along the lines of 'should my performance remain in line with what they'd seen then there would be no problem passing probation'.
It also probably depends on the type of mortgage you want too- we were going for a 100% which has stricter conditions relating to the length of time you're employed as well.
We didn't manage to sort it out at that time, though my partner is self employed so that may have affected the whole application too.
I'd say talk to a bank/ broker, there's some really helpful people on this site ...
HTH, Michelle.