Your opponent's insurers will want to test what you say to ensure that they only pay out the compensation to which you are entitled. They sometimes employ Private Investigators to follow and film clients. The likelihood of this is greater on the more serious injuries where a lot of compensation is being claimed and, most particularly, when someone is claiming an absolute, such as "I cannot walk" or "I cannot walk more than xx metres". People can, and are, caught out. We have received videos of people walking, running, playing sport or doing manual work when they have told us that they cannot do this. Obviously, this seriously undermines our clients credibility and they are left with some serious explaining to do.
The service of such a video on us can, in some cases, result in the entire claim collapsing and it has been known for insurers to then seek compensation themselves from Claimants because of their fraud. A favourite time for the Defendants insurers to carry out surveillance on a Claimant is when the Claimant is attending for an examination by their own (the Defendants) medical expert.
Surveillance by insurers is legitimate and they are entitled to do this. You should not fear surveillance so long as you have nothing to hide. Your opponents are only doing their job. We would much rather everyone was entirely honest and did not seek to exaggerate symptoms. Please note that people who are not being honest will be found out.