Hi Kevan
I realise you're in Wales but I don't think the rules and regs are that different, so this may be of help to you.
Page 86 of the "Essential Standards of Quality and Safety" Guidance doc via the Care Quality Commission:
http://www.cqc.org.uk/_db/_documents/Essential_standards_of_quality_and_safety_March_2010_FINAL.pdf
"All those involved in the care, treatment and support of the person who uses services:
–– cooperate with the planning and provision of care, treatment and support
–– have the documented plan of care available to them
–– have relevant information about the person who uses services available, where it has a direct bearing on the quality and safety of the care, treatment and support being delivered
–– record the key points of the care, treatment and support they have given
–– enable relevant information to be accessed in time to ensure that the needs of the person who uses services continue to be met."
[This reference was given to me by the CQC when I contacted them months ago now, on behalf of another TP member to ask the same question about access to care plans by close family.]
And if your Social Worker thinks you should be able to have a copy of the care plan/plan of care, s/he could perhaps arrange a "Best Interests" meeting with you present, to discuss the fact that it is in your MIL's best interests for you to know exactly what is in the plan. And in any case, a care plan should be put together in the first place with full involvement of a resident's close family supporters, especially where dementia is concerned. Often signed by the resident's close family member too, on behalf of the resident with dementia. And you need to be able to see the care plan, because it must be updated regularly.
There are some care homes that provide 'person centred care' and even 'relationship centred care' and who understand the need for family involvement in the care process. Here's just one provider that makes available care plans and daily reports that can be accessed online, and daily. Perhaps the way for the future?
http://www.cornfordhouse.co.uk/care-plans.html
The care plans and updates are not available to the whole wide world, of course, but secure access on an agreed "need to know" basis, in the best interests of the resident.
Wish you luck!