Hi Paul
The assessor has no more right to look round your home than any other visitor.
He/she is there to assess you, and see whether your answers to his/her questions accord with the answers you gave on your application form, nothing more. You might be asked to perform simple tasks to show whether you are capable of a range of movement, for example, but other than that you shouldn't expect to have to give any guided tour or anything else.
If you are asked to perform any task that you know will be painful, you can explain that you are unable - NOT unwilling - to attempt the task, It's always recommended that you have someone with you when the assessment takes place; this is to give you moral support and also as a witness to anything that you later consider inaccurate/unfair in the report. Other than that, be polite, don't answer questions that you aren't asked(!!) and don't take for granted that the assessor will have read your claim form!!
Good luck with the assessment