Can anyone give me a guide about how much I should be paying Stratstone for a five year service ?
The mileage is 23000 as I only drive 4500 miles per year.

Just ring around a few others and make a point of telling them you are comparing quotes....
Servicing costs are not fixed so you will be surprised...
And then tell your local dealer the cheapest quote....
Play them off each other like insurance quotes....never, ever accept the first price
Report back on your best quote  Visitor from fullfatrr.com
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I'll try doing that (though I'm not very good at bargaining)

The service is due in JUNE and I will report back.

The problem with hard bargaining is that I always think if I go too hard at it they will agree a low price but not do all the things necessary.

The questions to ask are
-Is the service as per the full LR schedule
-Are genuine LR parts being used (some use pattern parts and charge for LR price parts)
This way you can compare like for like.