has this problem happened before??

You could just be in a localised frost hollow or somthing of the kin (they can happen, even with the london climate), and the plant may think its still not quite spring. if fits got fat buds, looks okay and obviously isn't dead (you can use your finger nail to scrap a tiny peice of the bark away and check it is a nice pale/light green colour)give it a bit more time.
Some trees can remain dormant for years, sometimes decades before they bother to grow again (albeit not broad leaves), i'm guessing it'll be fine

YT

I have 2 acers, both of which I inherited when I bought this place so I don't know the exact names. I think my dark red-leaved one is probably one of the acer palmatum dissectum group, as it's low growing and spreads wide with fine leaves, and that's in leaf by now. My other acer, which is upright growing, has its yellowish-green leaves and tiny red flowers out now too.

A dark red acer just around the corner from me is is out in leaf. This is SE London. Sorry if this is not what you want to hear :( Normally I check deciducous trees/shrubs by gently scratching the bark and seeing if it's green (red?) underneath. If it's dry and dead...well...time to buy a new one... :(

Alcina

Personally I would have expected growth by now-especially if you live in London but as some suggest maybe it is just your tree and it's siting.
You dont indicate what form of jap maple it is-Disectum ones are more tender in general.
Also how old is the tree?
It's funny how trees grow. Across from my house there are 6 large oak trees. while 5 are dormant still-well showing no leaf from ground level one is covered in fresh green leaflets.

It's Acer palmatum. I don't know which cultivar, but it's not a low growing one and it's not dissectum. It grows in a sheltered spot close to the house. This is the highest point in the garden and we live on a hill. The tree is about 8ft tall. Every winter there's some dieback on the highest branches, which are taller than the fence and hedges. This time however the main trunk looks scorched by the wind. I snapped some branches off, they were not dead (yet). No signs of the buds bursting on this part of the tree though. A smaller branch on the bottom does look more promising, with fatter buds. Should I prune out the larger trunk/branch to give the smaller one a chance? Or just wait? The tree is probably 10 to 15 years old.

I have two japanese maples and one is placed in the front garden which is quite sunny. The leaves are opening but I would say it is not fully open yet. The one in my back garden (in shade a lot of the time and quite a few degrees colder than the front)has only just got it's buds and no leaves open yet. The one that you see round the corner could just be in a sunnier spot than yours. I would just wait a little longer to see.
Alison

As suggested leave your tree for now-I guess there is still a chance it will grow but things are not looking good. Even if you do prune back to this low branch is it worth it? Sounds to me like you may be having to get your spade out.
If you plant a new tree Acer Pal Bloodgood is a reliable variety.

i've got 4 varieties of acer & all 4 has got leaves although 1 has struggled to get the top buds out!

I've got one, three years old in a large pot on my decking. Think it is one of the acer palmatum dissectum group. It's still only 2 ft tall. The main problem is wind scorching which ruin the look of the tree very quickly, despite the fact that the tree is right in a corner between a five foot high solid fence and the rear house wall.

It is not unknown for fairly mature Acers to fail - the reasons are not always clear. There is a dieback disease which other posters have referred to, which usually causes no more than the extremities of branches to die back, but sometimes it seems to get more of a hold and cause large sections of a tree - or even the whole tree to die. I'm afraid it doesn't sound too promising in this case. Has it shown any signs of growth yet?

Good luck from America! My friend and I have been worried about our acer palmatums, too. I have about 50 and I've lost two for sure. Others have top dieback. Some are fine. We had a horrible cold spell after a very warm March. Everything was leafed out when the cold hit. I hope all ends up well. Char Rube