I want to try some bows on approval but the place I'm interested in using says I have to provide my own insurance (they insure the postage). Could anyone tell me how much this is likely to be for a week? It'll probably be 4 bows at around £500 each.

Most insurance companies insure individual items for up to a certain value provided they are in the home. I think mine is up to ?2000. So if you check your current contents policy you may be covered already. We also have an extra small addition (about ?20 per year I think) that insures items away from the home up to the same amount, so my girls violins, cellos are covered at concerts.

I could be wrong, but I don't think a conventional home insurance policy would cover items on approval, as they are not the property of the policyholder.
In a more general sense, I think it's better to insure instruments with a specialist company. They have a much more realistic idea of repair costs for musical instruments (often much higher than a normal home insurer would expect/pay out for) and they also provide financial compensation if the repair decreases the value of the instrument.
A couple of weeks policy with Allianz for the amount you mention would cost very little.

I pay £4 a month (for home and out of the home cover) for my double bass, violin and their bows (one each), total value of goods around £2000 to £2200
(with Allianz)

OK - thanks, all. The postal strike might have put paid to it for a bit anyway.