No hot water on combi boiler

Dear All
I have a Vaillant Sine 18. It was in the flat we bought - I've never looked at it just checked it had no Co emissions, but now it seems to have developped a problem.
Essentially it works fine while heating but it would not kick in when it should be at opening the hot water tabs. It started doing it with the shower. Since we are on the top floor I thought there'd be a problem with water pressure, but now it fails to kick in with any hot water tabs. If I switch on the main heating while the hot water tab is open it would also heat the water.
My knowledge of boiler is limited but I did a search and found a similar discussion:
http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&threadm 97101020405481149%40zetnet.co.uk&rnum&prev=/groups%3Fq%3Dno%2Bhot%2Bwater%2Bboiler%2Bpressure%2Bgroup:uk.d-i-y%2Bexpansion%26hl%3Den%26lr%3D%26ie%3DUTF-8%26oe%3DUTF-8%26selm%3D1997101020405481149%2540zetnet.co.uk%26rnum%3D10
Could it be an expasion vassel problem? When the boiler is cold the pressure reads about 1bar, and it goes up to 2.2bars when switching on the heating.
If so is it hard to fix it? Have I got to call an engineer or can I do it myself?
Many thanks in advance
Alex
PS: Quoting original message found on google:
From: [snip] Subject: Re: Boiler - Fix or replace ? Newsgroups: uk.d-i-y Date: 1997/10/10 Hi Ian
I work for BG,I suggest you get someone who knows about combi's to have a look at your valliant sine18. By what you are saying there doesn't seem much wrong with your boiler.There are a few checks to be made before a fault can be pinned down,don't let a guy guess the fault,it will cost you money and eventually he will find the faulty conponant after half the boiler is changed. If the expansion vessel has lost its charge,it will need to be pumped up to 0.7-0.9 bar.the easy way to check this without the use of a pressure guage is to check the pressure on the front of your boiler is at 1bar cold,then turn on your heating,if the pressure rises up past 2bar then the expansion vessel needs repressurising. The fault with no hot water usually lies with the changeover valve's wax capsual or valve not making the microswitches.
[snip]
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Alex wrote:

As per the posting you found the HW problem is not likely to be the expansion vessel esp if the CH is working ok. It more likely to be problem with the water flow sensor or divertor valve.
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Dear All
I posted what below some time ago to which Billr kindly replied.
Can anyone tell me the usual correct behaviour of the pressure in the system at rest, upon kickin in, and at max temperature. Many thanks. In this way I would exclude the possibility of the pressure in the system needing adjustment. Thanks
Alex
BillR wrote:

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On Thu, 06 Nov 2003 17:59:47 +0000, Alex wrote:

See Sealed system FAQ.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
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Many thanks!
Ed Sirett wrote:

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Many thanks!
Ed Sirett wrote:

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