Silicone spray on suspension arms is like trying to put a fire out by pouring petrol on it. It is a totally pointless endeavour and not only a waste of your time but a waste of money on a tin of perfectly good silicone spray.
You have one of two options dependent upon the state of your suspension:
1. You can disassemble your upper suspension arm assembly, clean, flush and then grease the upper arm bearings and then replace the assembly. This is only acceptable if your suspension arms are in good condition but have dry bearings.
2. Replace your upper suspension arms with new items (not the £13 buypartsbuy items if they are still in circulation). Be sure to heavily grease the suspension arms bearings.
Upper suspension arms are removed by doing the following:
1. Open the bonnet and undo the upper strut mounting bolts on the suspension tower top. I think there are 4 nuts each side.
2. Jack the car up and remove the front wheel(s)
3. Undo the large bolt that runs through the lower suspension arm and supports the shock absorber mount tuning fork type thing.
4. Undo the ball joint on the top of the upright that bolts to the upper suspension arm.
5. You can now with some wriggling lower the suspension strut down into the wheel arch, although you will not be able to remove the strut.
6. Next remove the top plate off of the shock absorber, you may need to remove the little rubber washers on the threads which can slow you down. There should then be enough room to wriggle this plate, although with upper arm attached to it, out of the wheel arch.
7. You will now see the offending bearings on the upper arm. Undo the bolt that runs through these bearings and top plate to remove the arm (note which side the nut / bolt is, as it will foul the spring if incorrect). Clean these bearings up with a detailing / toothbrush and flush all the muck out of them with WD40 and dry off. Then grease the hell out of both sides of the bearing and be sure to grease the mounting faces on the plate, just to make your life easier.
This should stop your annoying squeak – how long for depends upon the life left in your current suspension arms. It will prolong the inevitable for some time anyway.