Yet another noisy boiler

The noise problem has been going on for about 3 years now and I despair of ever getting it sorted. The boiler is a Wickes 65F which was in the house when we bought it. We first had the plumber out in April 2001 as it was making a lot of noise, he said it was kettling and put some sentinel 100 and 200 into the system, this sorted the problem and all was fine for about 1 year when the problem started again. My husband bought some sentinel 200 and added that to the system himself, again the noise stopped. We had occasional periods of noise which would last for a few days but then suddenly stop we have always assumed (probably incorrectly) that this was caused by bits of limescale in the system which would get stuck and then move. All was quiet until October last year when we had thermostatic valves fitted to all of the radiators apart from the bathroom, we had this done as we have a coal fire downstairs and like to switch the radiators off when this is lit. After a few days the noise started again but we decided to just ignore it. About 3 weeks ago I came home from work to hear that the noise had got much worse and now it sounded like banging and rattling come from the boiler, we got the plumber out again who said it was kettling. He again treated it with sentinel 200 and everything became instantly quiet, however after a week the noise has returned not banging but just the usual noise that we can hear whenever the boiler fires up and which continues for a few seconds after it goes off. The noise is hard to describe but it is loud and my ears seem to have become permantly tuned in to it. We never have the heating on in the morning as we would not need an alarm clock it would wake us up very quickly.

I think we must have more sentinel 200 in the system than water now and really don't want to get the plumber out again. Do you think we should try another chemical such as fernox boiler silencer. Any suggestions would be very greatly appreciated.

Have you drained any water to see whether there is any debris in the system? It may be there is loose debris in the boiler , and this will need removing.

Can air easily rise on leaving boiler? Or do the pipes drop?

When we drained some water off to add the sentinel, the water did not have any debris in it. I don't know what it was like when the plumber drained some off to put more sentinel in but hopefully he would have said something if it had lots of muck in it. As for the pipes, they run straight for quite a distance when they come out from the boiler. The plumber says the location of the boiler is bad as it is in the utility room in an extension at the side of the house, he says that it is too far away from the pump which is upstairs in the airing cupboard and that the water takes too long to pump through ( I do not have a clue what he is talking about ) He also says that the boiler is not have a very good design as the pipes going into and out of it are very small!
I trust the plumber as we have known him for years and never had a problem with him but I am just too embarrassed to ask him back again. Do you think we need a power flush and would that definatley cure the problem.?
Thanks for the replies I have received so far.