Focus 1800 Intermittent Idle Issue

My year 2000 UK Focus has developed an intermittent fault.
Normally it's when the engine is warm, and I've been travelling above
When I take my foot off the accelerator, the car will keep going at roughly the same speed. If I dip the clutch, the engine can race to as high as 4000rpm.
Could this be the ISCV I keep seeing references to?
thanks in advance Phil
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.