Keyboard misbehaving

Hello,
This afternoon, my 5 year old dell laptop (XPS, M1530) keyboard started adding a character from the row of numbers/characters to most key strokes on the top row of letters, with the central & right end keys unaffected. It comes out like this:
q = q1; Q = Q!
w = w2; W = W"
e = e3; E = E£
r = r4; R = R$
t = t; T = T
y = y; Y = Y
u = u7; U = U&
i = i8; I = I*
o = o9; O = O(
p = p0; P = P)
[ = [; { = {
] = ]; } = }

If I type numbers or characters, I get the letters from the row below
1 = q1; ! = Q!
etc with 5/% and 6/^ unaffected, as t and y are unaffected

It's as if the other keys have fused, and I get the pair of characters every time I hit any one of those keys. As you might imagine, it takes a long time and a lot of deleting to type any sentence containing top row keys without producing a jumbled and difficult-to-decipher result, eg 'top row keys' becomes 'to9p0 r4o9w2 ke3ys'.

I'd be grateful for advice!

Best wishes,
Wendy