Have noticed a leaking corner piece. On inspection it seems the corner piece does not quite fit very well with the straight bits either side, its slightly too short.

There has been some fascia fitted underneath, so it looks like the corner piece is now sitting further out.

Can i get a corner piece with slightly longer sides? so that it meets the gutters better as it will be a pain and a big expense to change both pieces of guttering.

Or any other solution?

Depends how much too short the gutter is; you could pack the gutter brackets nearest the corner off the fascia a few mm maybe, which will gain you - a few mm .

Seal the joints with silicone or gutter sealant.

Check that any rubber seals are properly in place and not dirty.

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One day it will all be firewood.

The brackets are all along the fascia so the whole thing is out the thickness of the fascia all around. If i pack them out more won't that mean i will lose mm's???

Its leaking because it won't join together not because of the seals.

Some makes of gutter do vary in size, take the old one with you to the builders merchants and plastic centres.
Check that the gutters only go into the unions to the marks as shown inside fitting, they are not fitted tight to allow for movement.
You might be able to slide the gutter length back a bit, last option get a union/joiner and clip it in the run .

For some unknown reason I thought it was an internal corner - but I guess it must be an external corner, just ignore me.

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One day it will all be firewood.

If you were to put a straight, in-line connector joint somewhere in each run of guttering that should gain you almost an inch of length on each run. Enough to get the corner piece on properly.

That sounds just what i need, do you have a link/picture to what one looks like?

Its square line style, probably floplast.

Have looked and the corner piece has clips either corner to clip to the straights.

Look in Screwfix page 505
All the square line is on that page.

Am i looking at using the Square Line Union Bracket 114mm White?

If so i would need 2 of these and some gutter to add in either side?

If you only need to gain a little bit of length just take down your existing run of guttering and cut it at the point where one of the brackets is. Install a union instead of the bracket, the two cut ends won't quite meet in the union - so the overall length of the run will be increased by about 10 to 20mm.

That should gain you enough extra to get the corner piece to fit properly.

Either cut about 300mm back on each section of gutter, fit the union, it has 2no screw holes, then fit the of cut of gutter in between the union and corner.

Wickes corners are longer in length, I had to replace a corner last week, customer got the corner from Wickes, it was about 40mm longer each side, I had to trim the gutter to get it on.

Handy to know i won't need any extra guttering.

One side will be easy, the other side is a long straight run and has a stop end siliconed to next doors so is there a way or technique to cut it in situ for that piece?

Sounds like a job for.......

As i haven't got one of them and its quite easy access to the corner i might take it up to wickes and see if theirs is much bigger as i am only a few mm out each side and that may work.

The tool Inky Pete showed is one of the best and handiest bit of kit I brought, although I got the Bosch Gop Blue, and thats what I used to trim back the gutter.