Being self employed and needing a car for business, I have always taken out additional insurance covering a guaranteed replacement car in the case of a accident. Twice I've had non fault accidents and in each case Drive Assist were named on my policy to supply the replacement car while mine was being repaired. As I drive a medium sized automatic, Drive Assist supplied the smallest automatics on their books, which were Mercedes E series, which didn't come cheap! In each case the at fault driver's insurance companies paid up without a quibble and I did not need to go to court to justify the expense, even though Drive Assist was acting on my behalf.
Had I relied on the repairing garage to supply the replacement vehicle, I was told that it would have been a manual VW Polo.
If the steering of you car is pulling to one side, I think that you should not drive it in its current state, but take up the replacement car offer instead if this has come through the other party's insurers. As Barmaid says, if you were to hire a car yourself, you may not get a full refund of the costs.
Have you informed your insurers, Marks & Spencer, of this accident, as their Claims Department should be handling the negotiations with the other party's insurers and Drive Assist. Don't go to a No Win No Fee solicitor!
The Office of Fair Trading will be looking into the cost of credit hire replacement as part of their wider investigations into the cost of car insurance in the new year, as this along with injury claims and fraud has caused the large increase in insurance over the last few years.