Sorry if this seems a bit techie, but below is the official response as to what happened with the Bernards Heath playing field which was dug up recently.
The short story is that they found actually that there were two different problems on the field.  There will potentially always be a danger of holes appearing.  All they can do is leave the earth mounds there so that if a hole appears it will dimple, rather than create a deep small hole.  This should make it safer.
The hope is that they will reseed the area properly, possibly to make an English grass meadow for example.

Now for the techie bit:
1. Nature of collapse and remediation works
Following the localised surface collapse, a sub soil investigation was arranged in August 2009, covering the entire field area and comprising:-
• 12 No bore holes to depths of up to 5m
• 71 No probe tests to depths of up to 12m
The tests indicated two distinct site issues:-
•    Solution features (swallow holes): generally to the south of the site. 2 were identified, including the original collapse. One contained fill, possible from previous infilling to at least 5m depth, the original hole deepened suddenly by 800mm during the investigation.
•    Weak fill with voids to the areas of the filled clay excavations, generally to the central and north areas of the site, denoted by areas of undulation across the field surface. Bore holes found fill depth ranged from about 2m to in excess of 4m. Probing found weak ground extended to at least 8m depth in one instance.
Options for remedial work to make the site safer rather than restoring the land to original contours, were put forward. The simplest and most cost effective has been implemented which is as follows:-
•    Pull back the top and sub soil of the area of the affected areas to a depth of around 1.5m
•    Compaction of the known or suspected weak areas.
•    Put down additional compacted layers of imported sub-soil to the excavated areas
•    Reinstate the surface with the original material pulled back material
•    The profile of the surface of the swallow hole to be mounded to a height in the order of 1.5m
•    The profile of the surface of the other areas to be mounded to a height in the order 0.6m
•    The excavations were rolled with a 20 tonne towed roller, sufficient to give surface and limited compaction to depth.
•    The additional material was introduced and compacted to provide a reservoir of soil should additional voids collapse and to help provide a ‘bridge’ across any potential features. The effect of this will be to allow any future collapse to be reflected by a dishing of the new surface rather than a potential sudden surface collapse.
•    Whilst there is likely to be some surface settlement the intention of the work and mounding is to reduce the risk of surface collapse should there be future settlement of the historic fill material or re-emergence of the swallow hole rather than allow the mounds to settle to the level of the original contours
•    The new surface has been graded to allow for mowing operations.
2. Improvement to the appearance of the field
•    Mowing operations will be resumed and, weather permitting, there should be two cuts before winter sets in and the route around the perimeter retained
•    The appearance of the mounded areas will be improved with the weeds either being cut or killed – advice on the best solution is being sought from the grounds maintenance contractor who has previously looked after the site
•    Options for the longer term planting and maintenance of the site will be produced with the aim of making the site generally more interesting (rather than just grass) and will be discussed with the Friends of Bernards Heath before implementation – these should be available in about two weeks
3. The future use of the field
The County Council has made no decision to change the current status of the site.