I know it's ok to mate a grafather with a grandaughter but is it ok to mate an uncle with a neice or is that too close?

I've done that one very successfully.

But you do need to know your bloodlines very well.

Line breeding is a valuable tool, but also a dangerous one.

As Lady Dazzle has said, it can be ok to do, if you know what you are doing. You can get something absolutely gorgeous, or absolutely hideous. It doesn't always work out as you expect.

In the one mating you are concentrating the grandfathers blood in the offspring, in the other you are concentrating the blood of two of the grandparents.  In both assuming it is the same Grandfatehr their influence would be the same, but in the latter you will have done the same with the Grandmother.  Whether it is too close depens on whether you get what you are aiming for and that the animals involved have the minimum possible bad qualities as breding thsi close will fix the traits making them very hartd to eliminate for generations, good or bad.

Will now go off and write it out to check I ahve got my percentages right, so don't take it as gospel :D

Right just wrote it down.

When mating a dog to a Grandparent the resulting offspring will have 62.5% of that grandparent in the pedigree.

A full uncle to a neice or Aunt with a nephew, will give you 32.5% of both the common grandsire and grandam, but the pups will have 65% of the same ancestry in this mating compared to the granparent one.

A half brother sister would give you 50% of the common ancestor, so the same as if he were the sire.

Two of my bitches are the result of half uncle to neice matings giveing the 32.5% influence of the common ancestor.  This was done by mating a bitch to her mothers half brother.