I obviously can't tell you what your problem is without doing a survey, but I am happy to give you some clues as to what to look for.
Firstly what is subsidence.....Subsidence is what occurs when support is removed from beneath foundations. This can occur for numerous reasons, for example the soil can collapse where there have been mine workings or a well, it can be eroded or desicated by water (say an underground stream), but the most common cause is drying out of shrinkable soils. There is every chance (being Greater London) that you have a clay sub soil and clay is a shrinkable soil.
Secondly how do you identify subsidence.....This requires both knowledge (knowing what clues to look for) and experience (the clues are just that, they aren't definitive). Cracks caused by subsidence tend to appear low down on the walls and the cracks will tend to go through the whole wall (not just be apparent internally or externally). As the movement progresses the crack will widen and get longer going upwards through the wall. The crack will be wider at the bottom than at the top and will often go through the bricks rather than follows the mortar line.
Thirdly, is subsidence a problem...More often than not, No. Subsidence is a bit like asbestos....people here the term and envisage disaster. Buildings move all of the time. The movements are induced by changes in the temparature of materials or by changes in the water content of materials. Movement, including soil movement, is frequently seasonal and cracks will open and close as the seasons change. Movement only becomes a problem when it is progressive and ongoing.....that is it keeps going in one direction and doesn't stop.
You have described cracks in your ceiling. It is unlikely that they are caused by subsidence. More likely they are just cracks in an old lath and plaster ceiling (which may need to be replaced or boarded over if it is sounding hollow) or movement of the plasterboards (which will produce cracks in straight lines and is purely aesthetic).
Cracks of 2mm wouldn't normally worry me. If they get to 4mm it may be worthwhile having a Chartered Building Surveyor take a look. Measuring the width of a crack is easy with a crack guage which you can buy here for less than £3.00 see
here Just click on the products button and scroll down.
The crack in the wall is more worrying. The "Upvc Windows" doesn't add up. If the original windows were not load bearing then removing them would not cause movement. If they were and lintels were installed when the new windows were then any settlement would be above the window. Quite commonly leaky drains cause the opposite of subsidence, which is called heave. In this case the extra moisture getting into the soil causes it to swell and lift the foundation. On the surface it sounds like this may be a possibility. The cure is to fix the leak and repair the damage to the wall in an appropriate way.
Charges made by surveyors vary considerably. For this sort of work I generally charge £250.00 to visit sites within reasonable travelling distance of my office, carry out an initial appraisal of a problem, make brief recommendations and confirm them in a letter. Although that is all that is required in many cases, some require further more detailed investigation and once I have seen the problem I am better able to advise what those things might cost. Most buildings insurance policies allow you to claim the cost of professional services related to a claim.
However, I have quite low overheads for a surveyor, so I would say that anything from £250.00 to £500.00 is probably reasonable. An inspection like this is unpredictable and I would allow half a day for travel, inspecting and writing a brief report. I certainly charge more than the £250.00 if it involves anything more than half an hours drive.
