It's a shield bug - harmless, but also known as stinkbug as it can squirt nasty smelling liquid if harassed or frightened. We mostly have green ones. This one is a forest shield bug.

I think its a sheild bug, though I'm used to seeing green ones. They are harmless as far as I'm aware, to both plants and humans

As Beattie says, it's a Forest Shield-bug (Pentatoma rufipes). Some species are carnivorous, but this one is a sap-sucker that feeds on trees, generally Oaks, but they will happily adapt to all manner of garden trees. Like many other sap-suckers they've also been proven to transmit plant viruses around.

Having said that they rarely occur in large numbers, are unlikely to have any noticeable effect on your garden plants are are really quite cute (well, they are to me anyhow!) so should really just be considered as part of the overall balance of your garden.

Finally, the nymphs of some species of Shield Bug are incredibly beautiful....like little jewels. Your bug has a rather less impressive juvenile stage though:

http://www.laurencecounter.com/Macro/Highgate-Wood/5407301_g4rdY/36/547566457_m3TQJ#547566457_m3TQJ-A-LB

I have loads of them !!!!! Would you like some more lol
Mostly have the green ones though. I must say they seem a lot bigger this year!!

Great - the Green Shield Bug (Palomena prasina) is by far the most common species, so that's prob what you have lots of!

We have the magnificent (and highly carnivorous & predatory!!) Troilus luridus in our garden.... the adults are bright red and the nymphs gold and metallic green:

http://www.gardensafari.net/pics/wantsen/schildwantsen/troilus_luridus_hs0_1089.jpg

So I think we'll stick with them thanks :-)

I WANT ONE!! We have lots of trees - maybe we've got them already?

You can have all of mine Beattie with pleasure!!!!! lol

Only if you've got Troilus luridus, Great! I've got green ones already thanks.

oh ok I;ll keep them then lol

You guys (and gals) are brill you know everthing thanks

Fascinating! I've been stabbed by a related Heteropteran Bug in the past Ilex :-)

Fractal - some of the aquatic ones, in particular, are pretty fearsome beasties...I do like them a lot, but try to steer clear of their mouthparts!!

I had one in the fuschias with a section of yellow and black stripes on the sides of its 'shell'. Does this mean a warning, like wasps, or nasty tapes used to fix warning A4 bits of paper on my bike saying, 'leave it here again and it will be removed.'.....I left a really old one there and it was removed!!!!!! free.

LOL I've heard that in some areas, if you want to get rid of something like a fridge you put it outside the door with a sign on that says "£10, please knock at no 35" and when you open the door again, it's gone!

We left a fridge freezer out front, near the pavement, having rung the council. The next morning the local 'totter' came ringing his bell and started loading it on his truck. We were very pleased. He said 'leave anything metal near the threshold on thursday for friday and I will take it.' A long way from shield bugs. I have seen about 3 different coloured bugs in the garden recently, perhaps the one with stripes is the one that bites?

I think all of the predatory species are capable of inflicting a puncture wound if they put their minds to it!

Speaking of striped Shield Bugs, check this one out:

http://www.richard-seaman.com/Wallpaper/Nature/TrueBugs/RedAndBlackStripedShieldBug.jpg

Fantastic!