Read this;
http://www.probate.uk.com/caveat_stopping_probate.html
This says:
"I have applied to the probate registry and just received notification that a Caveat has been issued, what can I do?
As no notice has to be given when a caveat is issued, it is often very surprising to Executors or Administrators to find that a caveat has been issued against an estate. Given this if you disagree with the Caveat, something called a Warning maybe served on the Caveator. This provides that an appearance to the caveat, must be entered at the Court within 8 days. The appearance sets out in summary the grounds as to why the caveat has been issued. If no appearance is entered, then an application can then be made to the probate registry for the grant of probate to be issued.
If an appearance has been entered, the caveat will remain in force indefinitely, until matters are finally resolved, this could be by either an application being made to the probate registry for a direction hearing or alternatively formal contested probate proceedings being commenced.
On account of these rules cases often arise whereby the testator the person making the will) hasn’t maybe signed the will in the presence of the witnesses, the signature does not appear to be genuine or the witness to the will is in fact a beneficiary."
I would be inclined to ask the SIL though- it may be something that can be cleared up between you