I'm not sure you'll like Silence (text by Harold Pinter). Edges strongly towards the avant garde. What are your feelings about Escalator Over the Hill? That might be the nearest reference point. On the other hand, if you have a yen for great British eccentrics like Robert Wyatt and Kevin Coyne, it's indispensable. 

I'm not sure if it's not a specialist taste (I like it though: I remember playing it in my Willesden bedsit. It's got a very seedy Willesden bedsit feel and I strongly related to it). I don't think collectors have latched onto Michael Mantler (they haven't latched onto Kevin Coyne beyond the Dandelion period, though Robert Wyatt has a cult following, I'm sure). 

A check on popsike indicates that the CD issue coupling two Mantler LPs, Silence and No Answer (starring Jack Bruce, and even more unlistenable: Sam Beckett text) fetches more than the original LP Silence. My hunch is that £15 is over the odds. 

On further investigation...

My records - based on an analysis of prices gained or not on eBay - show that within the past year a copy of Michael Mantler, Silence, didn't sell at the asking price of £15, whereas Michael Mantler, The Hapless Child (w Robert Wyatt, with Edward Gorey text, and more accessible than either Silence or No Answer, in my opinion) fetched £7.

There you go: I know the price of everything and the value of everything.