I have an old A.701A chef, but the push button release catch will not hold the top arm down in position.

That is really old, focus your search in the UK and auction sites selling second hand stuff.

Late but I had the same problem and fixed it on me tod so maybe it will help someone else. There are two stages of closing, it will shut at around 6 o clock but it's locked when it's around 8 o clock (it should click into this same position when fully open) I fixed mine by pushing it down harder than I was doing. If it won't lock after this then a spring is borked.

Can't refer you to a dealer there but it is likely that only the motor's brushes need to be replaced. This isn't a difficult DIY job and the brushes shouldn't cost more than ~ $20 US.
You could also remove the housing covering the motor and see if the commutator is visible. This is a segmented copper cylinder and shouldn't be completely blackened although some signs of black is normal. The brushes ride on this cylinder and if the brushes have become too short, it can cause arcing and smoke. Clean that surface before installing new brushes and they too may smoke and cause rough running until they are worn in. The brushes can be had many places so you will have to choose one convenient to you:
Kenwood Chef Brushes

There is a spring, item #4, which hooks on to the bottom of the control plate, item # 25, and pulls it back when you move the lever to off. Remove the rear cover and see if that spring is still connected to the control plate and to the lever arm, Item # 3. Here is the diagram:

http://shared.whirlpoolcorp.com/assets/pdfs/literature/Repair%20Part%20List%20-%208212369.pdf

I got service manual if any one want it.seamusria-at hotmail.com for kenwood Kenwood chef model A.701A.

Push down slightly on the head and then the button will go in further and the top will release. ((this answer after I just bought it home and was about to take it back to the store!!))

Yes,
DISCONNECT POWER
if you remove the top cover, you will find an overload group with a green and a red button on top, next to the contactor . One is test, the other is reset. On the side of the overload are two thumb wheels, one is auto/ manual, set this to auto, the other is -15%,0, +15%. If you are running very heavy batches and you want to push your motor harder, move it to +15, but it's not recommended. If it was set to manual, when it trips and cools down, it will not reset untill you press the reset button, in auto, once cool you an restart the mixer.

Okay, we just figured this out ourselves.
There is a little hole on the neck of the mixer, where the mixer motor meets the neck. While pushing the tilt release button, stick a small screwdriver into the hole (it will not go far) and pry the latch upward. While holding the latch up and the tilt button, have someone pull up on the motor while holding down the base. It might take a little muscle, but after that, the motor will be able to tilt back and forth like it did before it got stuck. If you are the one holding the tilt button and latch, please wach your finger so they don't get pinched when the motor swings up.

Look under the mixing head on the bowl side. There's a square "tab" that looks like the one on the back side of the machine. Pull it towards the bowl to release the mixer head from the base.

To open take off the top and open the three screws. Use light grease.