i would have a free wheel alignment check done & ask them to check the lower suspension arms whilst there as they wear &
can cause the symptons you talking about. for how much they cost i would replace both lower suspension arms & you will feel a hell of a diffrence. they cost about £30.00 each so worth it m8 if you can do them yourself.as said by mick85 its worth getting the steering rack looked at or atleast change the tie rod end as they maybe worn but do anywork like tie rods and lower suspension befor you pay out for wheel alignment. my local garage diagnose a some parts on my car needed replacing so tracked as best they could for me then i done the work & they redone the tracking for me for nothing the following week.
as btmaldon said its on the rear of the engine. i can usually lie under the car from the front offside tyre & get my hand on the filter & undo it but if it's hard to undo put it on ramps of axel stands and use a strap of chain type remover but when refitting use a little oil on the
new filter rubber seal & do it up by hand and dont use a tool or use the tool to only nip it up.