Cracks in plaster in conservatory!!

[macs]

We moved into our house at the start of the year and it has a conservatory built on the side, built in 2003. Quite large, approximately 16ft x 8ft. Dwarf wall about 600mm high all around with windows and doors on 3 sides, 4th side being external wall of the house. This wall has been plastered in the conservatory, as have the dwarf walls.

The plasterwork is cracking. I did notice some when we moved in but I put this down to just the plasterwork drying out as it gets quite warm in there. However, we have recently noticed some quite large cracks in the plaster on the main external wall of the house, in particular where the conservatory joins the house. The cracks start where the sill in cut into the plaster and extend inwards and slightly up. They are approx 1-1.5mm in width at the start and taper quite quickly to a hairline but continue for about 1m. We only noticed these as we have had something in front of them until now so cannot comment if they are getting worse. They are a bit crazed and do not follow the line of any brickwork. The cracks are not present on the other side of the wall inside the house. There are also some hairline cracks around the door from house to conservatory.

There are some tall conifers about 2-3 metres away and 3-4 metres tall, could it just be that because of the dry summer the trees have taken any moisture from the ground and the conservatory has subsided slightly and during the winter months the ground will be hydrated again and raise the foundations slightly to close the cracks up? Or is there something else at play here? Perhaps just the plasterwork drying more?

There is a slight opening of the join between the dwarf wall and existing house wall, approx 5mm. But that was there before I noticed the cracks internally.

I would like to know if I should fill the cracks and paint over so I cannot see them anymore? or is that just hiding from the truth?

We have marked on the extent of the cracks to see if they are getting worse.

Any help, pointers, tips, whatever will be appreciated.