I wanted to insure our higher value items (my watches, cameras and my wife's jewellery) and most general insurers had a maximum of £20K for such items. I needed more than that, and also went to TM March but they turned out to be quite expensive. I found the best thing was to wait until my home and contents renewal was due and then find an insurer that could offer home and contents, plus a much higher high value item limit.
I found one and I now have high value items insured up to £50K with a single item limit of a maximum of £15K and no requirement to produce all the valuations and/or receipts unless a claim is made. All items are also insured worldwide for any length of time away from home.
The nice thing is that this is all in one place, and the annual premium is less than my old home and contents and old high value items policy combined.
Send me a PM and I will provide the name of the insurer I went with.
Simon