I expect to receive a settlement as a result of a car accident and wonder if this will impact my lone parent payment, especially as a lot of the money will be needed for medical and dental treatments which I can't afford until I get the settlement
It is very much their business as there is no exemption from means assessment for insurance settlements - the money is treated as savings and assessed accordingly.
The assessment of savings can be reviewed regularly as you incur expenditure for medical treatment - once it can be shown that you've spent the money appropriately, the amount assessed against you will be reduced.
When carrying out a review of savings, SW check to make sure that money isn't being "hidden" (e.g. by transfers to another individual) to avoid a means test. If you can show that the money has been spent on items such as you've described (by keeping invoices/receipts for high-value items, or keeping bank statements which show payments to 3rd parties like the oil company), that should not be a problem.
I have been unable to find any reference to how I am allowed to spend my savings (well, settlement, really) in Social Welfare guidelines, or any reference to how often this assessment is reviewed. Is there any standard guideline I can access, or do they assess each case individually?
Here is what the guidelines on Means Assessment say:
(h) Withdrawals of capital not accounted for.
Large withdrawals of capital should be investigated - receipts should be requested to validate large sums spent. See "Property transferred" below re capital that is transferred to another person. Withdrawals that are not satisfactorily accounted for should be assessed.
There isn't a specific list of items, and no definition of what "large" is in the context of the above statement. This suggests that each case can be looked at on its own merits.